diff --git a/src-tauri/gen/android/app/src/main/AndroidManifest.xml b/src-tauri/gen/android/app/src/main/AndroidManifest.xml
index 8d005deb0..1ae80b4c2 100644
--- a/src-tauri/gen/android/app/src/main/AndroidManifest.xml
+++ b/src-tauri/gen/android/app/src/main/AndroidManifest.xml
@@ -13,6 +13,8 @@
         android:roundIcon="@mipmap/ic_launcher_round"
         android:label="@string/app_name"
         android:theme="@style/Theme.voltius"
+        android:allowBackup="false"
+        android:dataExtractionRules="@xml/data_extraction_rules"
         android:enableOnBackInvokedCallback="true"
         android:usesCleartextTraffic="${usesCleartextTraffic}">
         <activity
diff --git a/src-tauri/gen/android/app/src/main/java/com/voltius/app/VoltiusKeychain.kt b/src-tauri/gen/android/app/src/main/java/com/voltius/app/VoltiusKeychain.kt
index 5a0f16420..9d3cb77dd 100644
--- a/src-tauri/gen/android/app/src/main/java/com/voltius/app/VoltiusKeychain.kt
+++ b/src-tauri/gen/android/app/src/main/java/com/voltius/app/VoltiusKeychain.kt
@@ -1,8 +1,13 @@
 package com.voltius.app
 
 import android.content.Context
+import android.content.SharedPreferences
+import android.util.Log
 import androidx.security.crypto.EncryptedSharedPreferences
 import androidx.security.crypto.MasterKey
+import java.io.File
+import java.io.IOException
+import java.security.GeneralSecurityException
 
 /**
  * Secret storage backing Voltius' OS-keychain on Android.
@@ -16,7 +21,10 @@ import androidx.security.crypto.MasterKey
  * surface to these three static String-in/String-out methods so the JNI side stays trivial.
  */
 object VoltiusKeychain {
+    private const val TAG = "VoltiusKeychain"
     private const val FILE = "voltius_secrets"
+    private const val KEEP_QUARANTINED = 3
+    private val QUARANTINED = Regex("""^${FILE}\.xml\.(\d+)\.bak$""")
 
     /**
      * Hands the Rust keyring store the JavaVM + application Context. Must be called once at
@@ -26,14 +34,49 @@ object VoltiusKeychain {
     @JvmStatic
     external fun nativeInit(context: Context)
 
-    private fun prefs(ctx: Context) = EncryptedSharedPreferences.create(
+    private fun open(ctx: Context, masterKey: MasterKey) = EncryptedSharedPreferences.create(
         ctx,
         FILE,
-        MasterKey.Builder(ctx).setKeyScheme(MasterKey.KeyScheme.AES256_GCM).build(),
+        masterKey,
         EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
         EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM,
     )
 
+    /** A file this install's Keystore key can't decrypt is set aside and replaced by an empty store. */
+    @Synchronized
+    private fun prefs(ctx: Context): SharedPreferences {
+        val masterKey = MasterKey.Builder(ctx).setKeyScheme(MasterKey.KeyScheme.AES256_GCM).build()
+        return try {
+            open(ctx, masterKey)
+        } catch (e: GeneralSecurityException) {
+            quarantine(ctx, e)
+            open(ctx, masterKey)
+        } catch (e: IOException) {
+            quarantine(ctx, e)
+            open(ctx, masterKey)
+        }
+    }
+
+    private fun quarantine(ctx: Context, cause: Exception) {
+        val dir = File(ctx.applicationInfo.dataDir, "shared_prefs")
+        val file = File(dir, "$FILE.xml")
+        if (!file.exists()) throw cause
+        val taken = quarantinedStamps(dir)
+        var stamp = maxOf(System.currentTimeMillis(), (taken.maxOrNull() ?: 0L) + 1)
+        while (File(dir, "$FILE.xml.$stamp.bak").exists()) stamp++
+        val dest = File(dir, "$FILE.xml.$stamp.bak")
+        if (!file.renameTo(dest)) throw cause
+        // The file is already gone: this only drops the stale copy cached by the Context.
+        ctx.deleteSharedPreferences(FILE)
+        Log.w(TAG, "Could not decrypt $FILE, set it aside as ${dest.name}", cause)
+        quarantinedStamps(dir).sortedDescending().drop(KEEP_QUARANTINED).forEach {
+            File(dir, "$FILE.xml.$it.bak").delete()
+        }
+    }
+
+    private fun quarantinedStamps(dir: File): List<Long> =
+        dir.list().orEmpty().mapNotNull { QUARANTINED.find(it)?.groupValues?.get(1)?.toLongOrNull() }
+
     /** Returns the stored value for [key], or null if absent. */
     @JvmStatic
     fun get(ctx: Context, key: String): String? = prefs(ctx).getString(key, null)
diff --git a/src-tauri/gen/android/app/src/main/res/xml/data_extraction_rules.xml b/src-tauri/gen/android/app/src/main/res/xml/data_extraction_rules.xml
new file mode 100644
index 000000000..e83be5afb
--- /dev/null
+++ b/src-tauri/gen/android/app/src/main/res/xml/data_extraction_rules.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- The Keystore key that decrypts voltius_secrets.xml never leaves the device. -->
+<data-extraction-rules>
+    <cloud-backup>
+        <exclude domain="sharedpref" path="voltius_secrets.xml" />
+    </cloud-backup>
+    <device-transfer>
+        <exclude domain="sharedpref" path="voltius_secrets.xml" />
+    </device-transfer>
+</data-extraction-rules>
diff --git a/src-tauri/src/android_ctx.rs b/src-tauri/src/android_ctx.rs
index 8b2b6e8b1..62287b8bd 100644
--- a/src-tauri/src/android_ctx.rs
+++ b/src-tauri/src/android_ctx.rs
@@ -11,7 +11,7 @@
 
 use std::sync::OnceLock;
 
-use jni::objects::{GlobalRef, JClass, JObject};
+use jni::objects::{GlobalRef, JClass, JObject, JString};
 use jni::{JNIEnv, JavaVM};
 
 struct AndroidCtx {
@@ -59,7 +59,7 @@ pub extern "system" fn Java_com_voltius_app_VoltiusKeychain_nativeInit<'local>(
 
 /// Run `f` with an attached `JNIEnv` and the application `Context`. Returns `Err` (never
 /// panics) if `nativeInit` hasn't run or the attach fails. On a Java exception, describes it
-/// to logcat and clears it so the VM stays usable.
+/// to logcat, clears it so the VM stays usable, and names it in the error.
 pub fn with_env<T>(
     op: &str,
     f: impl FnOnce(&mut JNIEnv, &JObject) -> Result<T, jni::errors::Error>,
@@ -71,13 +71,51 @@ pub fn with_env<T>(
         .vm
         .attach_current_thread()
         .map_err(|e| format!("{op}: attach: {e}"))?;
-    f(&mut env, ctx.context.as_obj()).map_err(|e| {
-        let _ = env.exception_describe();
-        let _ = env.exception_clear();
-        format!("{op}: {e}")
+    let result = f(&mut env, ctx.context.as_obj());
+    result.map_err(|e| match take_java_exception(&mut env) {
+        Some(thrown) => format!("{op}: {thrown}"),
+        None => format!("{op}: {e}"),
     })
 }
 
+/// Clears the pending Java exception and renders it with its causes.
+fn take_java_exception(env: &mut JNIEnv) -> Option<String> {
+    if !env.exception_check().unwrap_or(false) {
+        return None;
+    }
+    let thrown = env.exception_occurred().ok();
+    let _ = env.exception_describe();
+    let _ = env.exception_clear();
+    let mut current = JObject::from(thrown?);
+    let mut parts = Vec::new();
+    for _ in 0..4 {
+        match throwable_text(env, &current) {
+            Some(text) => parts.push(text),
+            None => break,
+        }
+        let cause = env
+            .call_method(&current, "getCause", "()Ljava/lang/Throwable;", &[])
+            .and_then(|v| v.l());
+        match cause {
+            Ok(next) if !next.is_null() && !env.is_same_object(&next, &current).unwrap_or(true) => {
+                current = next;
+            }
+            _ => break,
+        }
+    }
+    let _ = env.exception_clear();
+    (!parts.is_empty()).then(|| parts.join(", caused by: "))
+}
+
+fn throwable_text(env: &mut JNIEnv, thrown: &JObject) -> Option<String> {
+    let text = env
+        .call_method(thrown, "toString", "()Ljava/lang/String;", &[])
+        .and_then(|v| v.l())
+        .ok()?;
+    let text: String = env.get_string(&JString::from(text)).ok()?.into();
+    Some(text.split_whitespace().collect::<Vec<_>>().join(" "))
+}
+
 /// Resolve an app class by dotted binary name (e.g. `com.voltius.app.VoltiusDownloads`) via
 /// the application class loader captured at `nativeInit`. Use this instead of
 /// `env.find_class`/`call_static_method(<str>, ..)` from threads Rust attaches itself (async
diff --git a/src/components/layout/SplashScreen.appLock.test.tsx b/src/components/layout/SplashScreen.appLock.test.tsx
index d6c08082d..087fba808 100644
--- a/src/components/layout/SplashScreen.appLock.test.tsx
+++ b/src/components/layout/SplashScreen.appLock.test.tsx
@@ -8,9 +8,10 @@ const h = vi.hoisted(() => ({
   available: true,
   exists: true,
   mode: null as string | null,
+  accountId: "acc-1" as string | null,
   launchLock: null as string | null,
   secret: "plain",
-  authProps: null as null | { isLocked: boolean; systemAuth?: boolean },
+  authProps: null as null | { isLocked: boolean; systemAuth?: boolean; vaultUnreadable?: boolean },
 }));
 
 vi.mock("react-i18next", () => ({
@@ -24,6 +25,7 @@ vi.mock("@/services/account", () => ({
   setAppLock: vi.fn(async () => {}),
   isServerMode: vi.fn(async () => h.mode === "server"),
   getAccountMode: vi.fn(async () => h.mode),
+  getAccountId: vi.fn(async () => h.accountId),
 }));
 vi.mock("@/services/appLock", () => ({
   systemAuthAvailable: vi.fn(async () => h.available),
@@ -35,7 +37,7 @@ vi.mock("@/stores/securityStore", () => ({
 }));
 vi.mock("@/services/vault", () => ({ getVaultStatus: vi.fn(async () => ({ exists: h.exists })) }));
 vi.mock("./AuthPage", () => ({
-  default: (p: { isLocked: boolean; systemAuth?: boolean }) => { h.authProps = p; return <div>auth-page</div>; },
+  default: (p: { isLocked: boolean; systemAuth?: boolean; vaultUnreadable?: boolean }) => { h.authProps = p; return <div>auth-page</div>; },
 }));
 vi.mock("./LogoBadge", () => ({ default: () => <div /> }));
 
@@ -47,6 +49,7 @@ beforeEach(() => {
   h.systemAuthUnlock = false;
   h.exists = true;
   h.mode = null;
+  h.accountId = "acc-1";
   h.launchLock = null;
   h.secret = "plain";
   h.authProps = null;
@@ -154,3 +157,19 @@ test("a declined login on a cloud account with no vault file yet still gets the
   await advance(2000);
   expect(h.authProps).toMatchObject({ isLocked: true });
 });
+
+test("a vault file whose account id is gone from the keychain goes to the unreadable panel, not the password form", async () => {
+  h.autoLogin.mockResolvedValueOnce("declined");
+  h.accountId = null;
+  render(<SplashScreen onReady={() => {}} />);
+  await advance(2000);
+  expect(h.authProps).toMatchObject({ isLocked: true, vaultUnreadable: true });
+});
+
+test("a vault lock over a vault whose account id is gone goes to the unreadable panel", async () => {
+  h.lock = "vault";
+  h.accountId = null;
+  render(<SplashScreen onReady={() => {}} />);
+  await advance(2000);
+  expect(h.authProps).toMatchObject({ isLocked: true, vaultUnreadable: true });
+});
diff --git a/src/components/layout/SplashScreen.keychainHint.test.tsx b/src/components/layout/SplashScreen.keychainHint.test.tsx
index 12fa76625..6236b57eb 100644
--- a/src/components/layout/SplashScreen.keychainHint.test.tsx
+++ b/src/components/layout/SplashScreen.keychainHint.test.tsx
@@ -17,6 +17,7 @@ vi.mock("@/services/account", () => ({
   setAppLock: vi.fn(async () => {}),
   isServerMode: vi.fn(async () => false),
   getAccountMode: vi.fn(async () => "local"),
+  getAccountId: vi.fn(async () => "acc-1"),
 }));
 vi.mock("@/services/appLock", () => ({ systemAuthAvailable: async () => false, lockOnLaunchIfIdle: async () => null }));
 vi.mock("@/services/vault", () => ({ getVaultStatus: vi.fn(async () => ({ exists: true })) }));
diff --git a/src/components/layout/SplashScreen.tsx b/src/components/layout/SplashScreen.tsx
index 4842cb858..94ac314b6 100644
--- a/src/components/layout/SplashScreen.tsx
+++ b/src/components/layout/SplashScreen.tsx
@@ -8,7 +8,7 @@ import { useFolderStore } from "@/stores/folderStore";
 import { useSnippetStore } from "@/stores/snippetStore";
 import { useSnippetFolderStore } from "@/stores/snippetFolderStore";
 import { usePortForwardingStore } from "@/stores/portForwardingStore";
-import { autoLogin, getAccountMode, getAppLock, isServerMode, setAppLock } from "@/services/account";
+import { autoLogin, getAccountId, getAccountMode, getAppLock, isServerMode, setAppLock } from "@/services/account";
 import { lockOnLaunchIfIdle, systemAuthAvailable } from "@/services/appLock";
 import { secretState } from "@/services/vaultSecret";
 import { useAppLockStore } from "@/stores/appLockStore";
@@ -44,9 +44,13 @@ function keepSwitcherFresh(): void {
   saveCurrentAccount().catch((e) => console.warn("[splash] could not save this account to the switcher:", e));
 }
 
+type UnlockPhase = "auth-locked" | "auth-unreadable" | "auth-first-launch";
+
 // An account can predate its vault file: a cloud one re-derives from the server, a local one never wrote a secret.
-async function hasAccountToUnlock(): Promise<boolean> {
-  return (await getVaultStatus()).exists || canLockVault(await getAccountMode());
+// A vault whose account id is gone from the keychain (restored without it) can't take a password.
+async function unlockPhase(): Promise<UnlockPhase> {
+  if ((await getVaultStatus()).exists) return (await getAccountId()) ? "auth-locked" : "auth-unreadable";
+  return canLockVault(await getAccountMode()) ? "auth-locked" : "auth-first-launch";
 }
 
 export default function SplashScreen({ onReady }: Props) {
@@ -62,6 +66,12 @@ export default function SplashScreen({ onReady }: Props) {
   const setStep = (id: string, status: StepStatus, label?: string) =>
     setSteps((prev) => prev.map((s) => s.id === id ? { ...s, status, ...(label ? { label } : {}) } : s));
 
+  const showUnlockPhase = (next: UnlockPhase, lockedLabel: string) => {
+    if (next === "auth-unreadable") setStep("vault", "error", t("layout.splash.vaultUnreadable"));
+    else setStep("vault", "done", next === "auth-locked" ? lockedLabel : t("layout.splash.firstLaunch"));
+    setPhase(next);
+  };
+
   useEffect(() => {
     async function init() {
       setStep("init", "running");
@@ -75,9 +85,7 @@ export default function SplashScreen({ onReady }: Props) {
         const sealed = (await secretState().catch(() => "none")) === "sealed";
         setSystemAuth(sealed || (useSecurityStore.getState().systemAuthUnlock && (await systemAuthAvailable())));
         try {
-          const locked = await hasAccountToUnlock();
-          setStep("vault", "done", locked ? t("layout.splash.vaultLocked") : t("layout.splash.firstLaunch"));
-          setPhase(locked ? "auth-locked" : "auth-first-launch");
+          showUnlockPhase(await unlockPhase(), t("layout.splash.vaultLocked"));
         } catch {
           setStep("vault", "error", t("layout.splash.vaultCheckFailed"));
           setPhase("auth-first-launch");
@@ -110,9 +118,7 @@ export default function SplashScreen({ onReady }: Props) {
 
       // autoLogin failed — check if a vault already exists (locked) or first launch
       try {
-        const locked = await hasAccountToUnlock();
-        setStep("vault", "done", locked ? t("layout.splash.vaultFound") : t("layout.splash.firstLaunch"));
-        setPhase(locked ? "auth-locked" : "auth-first-launch");
+        showUnlockPhase(await unlockPhase(), t("layout.splash.vaultFound"));
       } catch {
         setStep("vault", "error", t("layout.splash.vaultCheckFailed"));
         setPhase("auth-first-launch");
diff --git a/src/services/account.ts b/src/services/account.ts
index c97240dcb..1a949f651 100644
--- a/src/services/account.ts
+++ b/src/services/account.ts
@@ -587,6 +587,10 @@ export async function getAccountMode(): Promise<string | null> {
   return keychainGet("mode");
 }
 
+export async function getAccountId(): Promise<string | null> {
+  return keychainGet("account_id");
+}
+
 export async function getCurrentUserEmail(): Promise<string | null> {
   return keychainGet("email");
 }