A "Gamemode Error" appears whenever I open the spawn menu

Recently, after the second Achievement-adding update, my Gmod starting acting strangely. Whenever I opened the Spawn Menu, the below error message will appear. In addition, a second error message will appear if I were to spawn something out of the menu. Here’s a picture of each error.

Any ideas on what is going on, and how I could go about fixing it? It’s rather annoying. Thanks.

Anything? I’ve been looking online for an answer, but alas, have found nothing that could help. ANyone else had this issue?

If you are in singleplayer try deleting your sandbox folder and restart GMod.

Remove achievements addon.

I had this problem. But for me it dissapered when i got the “Bad Coder” Achievement.

Ah, I completely forgot I had that little bugger on. Thanks!