I have been looking into this a very long time, and as always the log ends with UnityEngine.Object.FindObject...
@tarynkelley have been looking into It too.
Seems like anything that is on timer, or on a thread causes the server to have a nice crash popup. (Player.Disconnect calls UnityEngine.Object.FindObject)
This has been resolved in a higher UnityEngine version, @Snake even found the changelog of It before.
Some of the plugins use a timer to handle checks later, and even disconnect players.
I have been thinking on several stuffs, how could I actually make It safer, but I can't come up with anything useful. I have been thinking on raising a hook, but I have been wondering if the hook would be running under a thread too.
The guy who gave the answer also shows the usage of the delegates, which we use for calling our hooks.
I will see if that would work or not