I guess we need a bit more info than that. The screenshots would definitely help. Has it happened only once? Are you running any InSim addons (AIRIO, CruiseMods, ...)?
We have 4 server running, only 1 server is causing problems. Could it be a Port interference somewhere? I've FileZilla Server installed and it uses 100 ports~ for file transfer (not only 21). So it might be using the port of the server.
The server is running an i7 920 and CPU usage is always about 15%~, so it's not the problem.