the following only applies after you have connected to a server that runs a mod and therefore have the mod downloaded:
basically everytime you try to join a server, that is running a mod, you are either forced to load the mod yourself
/fs_game mods/MODNAME
and do
/vid_restart_safe
and then connect or you have to delete the mod in AppData/..
if you do not do this your game will crash on attempting to do the vid_restart I assume.