Derma not working

My Gmod gamemodes aren’t working. I’m constantly spammed with the same Lua error

[ERROR] lua/includes/modules/gamemode.lua:86: attempt to index upvalue 'hook' (a nil value)
  1. Call - lua/includes/modules/gamemode.lua:86
   2. GetDefaultSkin - lua/derma/derma.lua:177
    3. GetSkin - lua/includes/extensions/client/panel.lua:354
     4. GetSkin - lua/includes/extensions/client/panel.lua:349
      5. unknown - lua/derma/derma.lua:199

and the derma menus of my gamemodes don’t work in singleplayer. For example in sandbox I can’t spawn any props
because the Q menu isn’t opening in the first place. I already verified the integrity of the game cache, so what now?

Might have to do a fresh install of Gmod.