Multiverse error spam

Hello, I have a problems with hosting a multiverse.
I can start my multiverse but when someone tries to join me, he get's stuck in a lobby.
I looked into a console and it was spamming this error:

An error has occurred: ArgumentNullException: Argument cannot be null.
Parameter name: hostNameOrAddress in: System.Net.Dns.GetHostEntry (System.String hostNameOrAddress)
Please send a bug report!

What I tried:
Reistalling the game
port forwarding 7777