Cannot load any custom maps

All the default gmod maps work fine, but every time I try to load a custom map I get this error:

I’ve tried reinstalling, defragging, verifying, and just about anything else I could think of but nothing helps.

Where did you get Garry’s Mod?

Through steam years ago… this is a new problem

Try this: http://wiki.garrysmod.com/?title=Cleaning_GMod

Nope… Still nothing