Issuing /kick /ban to any player always freezes the server and it's is not recoverable (it leads to a crash by timing out). You can reproduce it easily by kicking / banning yourself from the server program in game.

Tested on myself and on another player. Freezes the server every time if issued in the game. Works fine if issued in the console.

Tested on 19w09a and it was working fine. New bug starting from 19w11b (and probably 19w11a).

Crash report joined. Involving method sun.misc.Unsafe.park().