GMod Server crash when user connects

[KPDGaskell]

Hello, I'm having some trouble with a small Garry's Mod server I run. Normally it would run fine, but when a certain user joins (a good friend of mine) the server crashes.

Here's the error message we get:

Client "[Censored Username]" connected <[Censored IP]>
threadtools.cpp <2574> : Assertion Failed: Illegal termination of worker thread ' CNet Encrypt :0'
threadtools.cpp <2574> : Assertion Failed: Illegal termination of worker thread 'SteamFindServerThread'
threadtools.cpp <2574> : Assertion Failed: Illegal termination of worker thread ' IPCServer::m_ThreadClient'

So basically, what's the problem and how do I fix it?