Try shutting down server, renaming gmod/data/ULib/bans.txt => corrupt_bans.txt
Restart server, try again.
3.4 had a known bug with banning players with slashes in the name....corrupted ban (and users) files.
Going to SVN would only keep it from happening again, not delete the actual file.
Also, searching for ban crash from the main front page of the forums would have found other postings like this.