Crashing When Loading Maps

Okay, in the wake of the recent Gmod updates, I’ve been experiencing two very serious problems.

First, when I load the game, the screen becomes completely black, except for the in-game text.

Second, when I try to load any map, even Flatgrass, I’m presented with the following error:

Engine Error
failed to lock vertex buffer in CMeshDX8::LockVertexBuffer

This annoys me because I have several pose ideas in my head and I can’t do any of them.

Any help would be greatly appreciated.