diff --git a/apps/files_external/js/gdrive.js b/apps/files_external/js/gdrive.js
index eb626e6ce127..333c906c992b 100644
--- a/apps/files_external/js/gdrive.js
+++ b/apps/files_external/js/gdrive.js
@@ -1,6 +1,6 @@
 $(document).ready(function() {
 	var backendId = 'googledrive';
-	var backendUrl = OC.generateUrl('apps/files_external/ajax/oauth2.php');
+	var backendUrl = OC.generateUrl('/index.php/apps/files_external/ajax/oauth2.php');
 
 	function generateUrl($tr) {
 		// no mapping between client ID and Google 'project', so we always load the same URL
diff --git a/apps/files_sharing/js/public.js b/apps/files_sharing/js/public.js
index 8447f2a6bc36..ba09beaffb81 100644
--- a/apps/files_sharing/js/public.js
+++ b/apps/files_sharing/js/public.js
@@ -228,7 +228,7 @@ OCA.Sharing.PublicApp = {
 				urlSpec.x = Math.ceil(urlSpec.x);
 				urlSpec.y = Math.ceil(urlSpec.y);
 				urlSpec.t = $('#dirToken').val();
-				return OC.generateUrl('/apps/files_sharing/ajax/publicpreview.php?') + $.param(urlSpec);
+				return OC.generateUrl('/index.php/apps/files_sharing/ajax/publicpreview.php') + '?' + $.param(urlSpec);
 			};
 
 			this.fileList.updateEmptyContent = function () {
diff --git a/apps/files_trashbin/js/filelist.js b/apps/files_trashbin/js/filelist.js
index db0689766169..12ae829538c1 100644
--- a/apps/files_trashbin/js/filelist.js
+++ b/apps/files_trashbin/js/filelist.js
@@ -275,7 +275,7 @@
 		},
 
 		generatePreviewUrl: function(urlSpec) {
-			return OC.generateUrl('/apps/files_trashbin/ajax/preview.php?') + $.param(urlSpec);
+			return OC.generateUrl('/index.php/apps/files_trashbin/ajax/preview.php') + '?' + $.param(urlSpec);
 		},
 
 		getDownloadUrl: function() {
diff --git a/changelog/unreleased/41743 b/changelog/unreleased/41743
new file mode 100644
index 000000000000..9701163e5eb6
--- /dev/null
+++ b/changelog/unreleased/41743
@@ -0,0 +1,18 @@
+Bugfix: Request legacy ajax endpoints through the front controller
+
+The front controller rewrite only forwards a request to index.php when the
+requested path does not exist on disk. Five javascript call sites requested a url
+that was itself a real file, so the web server executed the script directly,
+without the bootstrap index.php would have performed, and the request died with a
+fatal error (HTTP 500): changing the personal language, the share dialog e-mail
+lookup, the Google Drive OAuth entry point, and the trashbin and public link
+preview thumbnails.
+
+All five now build their url with an explicit /index.php/ prefix so that the
+request no longer matches a file on disk and reaches the router. The prefix has
+to be part of the url literal because OC.generateUrl() omits /index.php when
+mod_rewrite is active, which is how four of the five came to request the shadowed
+url in the first place.
+
+https://github.com/owncloud/core/issues/41740
+https://github.com/owncloud/core/pull/41743
diff --git a/core/js/sharedialogmailview.js b/core/js/sharedialogmailview.js
index a47c867d07d1..d7fa155c9474 100644
--- a/core/js/sharedialogmailview.js
+++ b/core/js/sharedialogmailview.js
@@ -208,7 +208,7 @@
 						if (this.xhr != null)
 							this.xhr.abort();
 
-						var xhr = $.get(OC.generateUrl('core/ajax/share.php'), {
+						var xhr = $.get(OC.generateUrl('/index.php/core/ajax/share.php'), {
 							'fetch' : 'getShareWithEmail',
 							'search': query.term
 						}).done(function(result) {
diff --git a/settings/js/panels/profile.js b/settings/js/panels/profile.js
index 6c181a3d5f87..22c9426f1103 100644
--- a/settings/js/panels/profile.js
+++ b/settings/js/panels/profile.js
@@ -213,7 +213,7 @@ $(document).ready(function () {
 		// Serialize the data
 		var post = $("#languageinput").serialize();
 		// Ajax foo
-		$.post('ajax/setlanguage.php', post, function (data) {
+		$.post(OC.generateUrl('/index.php/settings/ajax/setlanguage.php'), post, function (data) {
 			if (data.status === "success") {
 				location.reload();
 			}