Mac permissions

I recently updated the app (12 Sep) and I haven't been able to change the filenames since. This is the error I'm seeing

I've tried opening the folder using ⌘O and also opening the app with root (sudo open "/Applications/", and also changing the permissions of the files themselves.

Please advise!

Have a look at this thread and see your problem is similar:

Also, a search for "secure bookmarks" in the forum may lead to more threads.

Can you try this command via while Mp3tag being closed:

defaults delete app.mp3tag.Mp3tag SecurityScopedBookmarks

After that, load the folder (not the individual files) via ⌘O and see if the issue persists.