Can you:
Close gmod
Open your garry'smod folder
remove these folders:
Start gmod
Enter the command Cow posted
..and then try joining again?
I'm having the same problem, the 'vgui_allowhtml 0' command works but I can't seem to connect to the HL2RP city server, and I just tried deleting those folders, nothing changed except for the fact it re-downloaded the cache and it just crashed again after that.
EDIT: I can connect to any other server perfectly fine, it just seems to be the city server I can't connect to.