internal exception: java.IOException: an existing connection was forcibly closed by the remote host

I was trying to join a new server because the old one got a new host anyway I couldn't and a message came up saying: "internal exception: java.IOException: an existing connection was forcibly closed by the remote host" please fix this im a head mod on the server and without me the owner and co owner cant control the server's vast amount of players. also when I hover over the connection bar it says the server is out of date!

Thank You