Garrys Mod crashes when I try to connect to a server

[Scotsheltie]

Hello everyone I am running Mac OSX and I can get on to other servers its just this one server when it says: Downloading gm_bigcity.nav it gets to the last dot in the loading bar and crashes then I get this error (failed to lock index buffer in CMeshDX8::LockIndexBuffer). Please help someone! I was playing on this server this morning and now I get this error. I have tried uninstalling and reinstalling garrys mod and deleting local content made sure I was not in Dev and disabled/enabled all my addons.

[Rubat]

[url]https://support.steampowered.com/kb_article.php?ref=9404-WRDH-5396[/url]