Running a vanilla server on Windows.
Problem: My friend's game keeps crashing when connecting to my server. None of the other players are experiencing this.
His game is legal and can play on the bigger servers just fine. So I think it's a problem with connecting to my smaller private server.
I've asked him to include the default port in the server address box and it hasn't helped.
I've also asked him to check his client-side folder for crash reports and he has none.
This also happened to the same player when I was running 1.9 as well. (I didn't host a 1.10 or 11 server :P)