I looked at the server directory, emptied it, and compared what happens when it's empty. The new cache files were created with a different user than what was there before. So as I suspected, someone had accidentally changed the ownership of the cache files to a different user, OR you recently moved to a new server, and you copied the cache files from the old server. If you have an automated script that changes file owners, then you should update it so that it does not touch this directory. If this happens again, then you must manually delete the contents of the directory (not the directory itself!) -- the cache may not have permission to do it itself.
This error is not a bug, and it is good that you see it in the logs because it alerts you that the file permissions of the directory were changed after the cache was already made.
You should not have to do anything right now. I have already emptied the cache and it has started regenerating with the correct permissions.