Garry's Mod Problem: failed to lock vertex buffer in CMeshDX8::LockVertexBuffer

Hi Guys, i’ve got the following problem:

Iam getting this error when i’m trying to connect to my DarkRP Server. It’s running the map rp_rockford_v1b with the usual addons… tdmcars etc.

I’ve tried to set -dxlevel commands in the startup options, check for errors, updated all drivers, reinstalled it.

My Setup:
Intel Core i7 3770k @ 4.5 GHz
16GB DDR3 2133 MHz RAM
Samsung EVO 850 256GB SSD
4 TB SATA6 SAMSUNG HDD
ASUS Geforce GTX 770 II OC
Windows 10 Pro 64bit

DirectX (dxdiag) Version:

If I run the Game in Singleplayer, I have no problems, works very fine, 20-30 other Players can connect without any problems.

Would be awesome if someone can help me!

Thanks

No Ideas?