Server can only retain 1 player in Stronghold/End Portal in the overworld after defeat the Ender Dragon

Hi, we have 2 players here, after we killed the Ender Dragon and go back overworld through the end portal, all players are disconnected from server. Only one player can successfully re-enter the Stronghold and another would disconnect again before the world load completely. It displays "{user} lost connection: Timed out" in server console. But it doesn't looks like network problem.

I tryed restart my server and it still appears. But if the only player leave that Stronghold/End Portal far away, another player can login again. Btw, there is nothing wrong before we defeat the Ender Dragon.

Sorry for poor English, and thank you.