Solution 1: Close Gmod down, go to Steam\SteamApps\common\GarrysMod\garrysmod\maps\graphs and remove the rp_bloc42_zarp.ain file and try to join again.
Solution 2: Close Gmod down, go to Steam\SteamApps\common\GarrysMod\garrysmod\download\maps and remove rp_bloc42_zarp.bsp and try to join again
If I come up with any new ideas, I'll tell you, but for now, I've no idea what causes it to happen, sorry.