I guess i have a similar problem here. Vers: Experimental 0.57.128120(Happened in the 2 previous versions too) Server: doesn't seem to matter. PC: CPU: intel I5 3570 GPU: Geforce 650 ti RAM: 16 GB Mainboard: Gigabyte Z77-DS3H Streaming Program: Open Broadcaster Software Internet: 16 Mb/s down, 1 Mb/s up
Video Memory is set to 2048mb, which should be ok. All other settings more or less to "normal" or "medium". Newest drivers gpu wise.
Added a screenshot of the error message.
Problem occurs verry randomly. Sometimes it happens 10 secs after login, sometimes after half an hour playtime. Always happens ingame. Never in the startmenue or serverlist e.g.
Frame freezes, game crashes and window with errormessage shows up.

Its not the usual "get your legs wrecked on the stairs" glitch. Its a wall you can actually walk through. No harm. I'll add pictures when i have the opportunity asap.