People cannot open the Q menu on my server.

Ever since I got Ultra Light RP, my players have not been able to open their gmod menu, I uninstalled it and it still doesn’t work.

They get this message:

ERROR: GAMEMODE:‘OnSpawnMenuOpen’ Failed: sandbox\gamemode\spawnmenu/spawnmenu.lua:50: attempt to index global ‘g_ContextMenu’ (a nil value)

try checking your addons folder, or try a vanilla Gmod

This error is created by the non updated unofficial achievement addon.

What is a vanilla GMOD? (sorry to sound noobish)

and how to fix it?

Vanilla means original, in short just reinstall your Garry’s mod and don’t download any modifications.