I just had the same issue with a 403 Forbidden on the cached version of the stylesheet only. And similar to nrohilla, most of the files in /wp-content/w3tc/min/ were set to 640 permissions. I manually changed them to 644 through FTP and everything worked again.
I agree. Ask for a donation. Put in an annoying registration reminder. But don’t put ads on people’s sites. Deleted.
