Did you do anything to your server immediately before this started? New addon?
New groups/any other modification?
The only thing I can think of quickly off the top of my head is that the file we use to store reasons, banners and temporary bans has grown corrupt.
I forget the exact location of the file, but look for it in gmod/data/ULX, something like temp_bans.txt or something like that.
Once you find it, try renaming then restarting your server.
You'll lose any temporary bans you've set, and the information of who banned who for what reason, but you'll keep your permanent bans. If renaming it doesn't work, you can rename it back after deleting the one that gets recreated.