Players can only log in for seconds, then lose connection and can't log in again

Haven't found this bug posted here yet, if it has been, please forgive me.

Players can only log in once, then lose connection and afterwards can't log in at all (io.netty.hander.timeout.ReadTimeoutException). A server restart returns to the state where they can log in again for a few seconds.

edit: Players are still shown as online in the server selection screen.