AC is sometimes over-sensitive to permissions issues. I've had issues when files on the server have full r/w permissions to both Owner, Group and All, but AC still refuses to comply because the Owner of the file has changed.
I regularly use BatchMod to allow permissions for all on our common project file server (OSX), so that no backups are being hindered by a file being locked or otherwise not allowed. Now I take care never to change owner of the files. Since this, the issue has disappeared. In Linux, I guess you'd have to do this witth some terminal command.
I'm no Unix guru, so this is all I can think of.
AC4.1-AC24SWE-25INT; OSX11.5; MP5,1+MBP16,1