opendkim is in group 80

[quwax]

Hello,
I run an old elCapitan-Server and re-set it up with the help of macports and ccompile with current binaries.
Thus I abandoned amavis which was doing signing before and compiled and installed openDKIM.

What ever I do as long as RequireSafeKeys is set to true, openDKIM will die with this error-message: _opendkim is in group 80 which has multiple users (e.g ... pointing in a wrong direction.
After some digging I found out, the actual error should be: key_data is not secure.

The problem I have is, I can't get the key secure.
I created new keys, set the permissions like this:
Permissions for the key-folder (and the content) are set to: drwx------ 4 _opendkim _opendkim 136 27 Apr 02:09 keys
without success (I tried that several times).

So either I miss here something, I don't understand or this is simply a bug and probably only happening on a small number of OSes, like old macoses.

How can I fix RequireSafeKeys= true?
Thank you