When you add a player with long nickname (not really existing) it crashes the server and kicks all the players with the message:
"Internal exception: Java.io.IOException: Received string length longer then maximum allowed (18 > 16)".
Then players also can't connect back.
Confirmed. However, this only happens when the score is being displayed (eg. in 'list'). Adding the invalid player does not disconnect players, displaying it does.