Engine Error

Client Lua Stack Leak [3]!
0> UNKNOWN (table)
1> 2 (string)
2> 2 (string)

Then game crashes, it pops up after I join a server. Any ideas on how to fix? I have already tried verifying my game cache.

It seems to be a known problem. Try to move all your addons out of Garry’s Mods and retry again.
If it fails again, then, report the problem.