Can't Connect to Server (Modern Skyblock 2)

Hello,

I've experienced a Problem I couldn't find any solution for so I write this post here.

My Problem: My friend runs a Modern Skyblock 2 Server and everytime I tried to join I get the message: Connection Lost

Disconnected

Or I get this message: Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet

Ingame-message in the picture!

Often I get the "Disconnected" Message and the other error message is out of the log and is a bit uncommon.

What I tried: I tried disabling my Firewall, but when I disable it nothing changed.

We also tried reloading the Modpack and the World and it didn't helped as well.

I tried to play the server world in Singleplayer wich worked fine, I didn't get any Problems there.

Quote from Roman

Your friend is able to join your server without any problems?

Did you already try to reinstall the modpack on your client?

Yea my friends do not have any problems to join and I also tried reinstalling the modpack in the Curse/Twitch client but it didn't worked for me

Whats a DynIP or how do I set it up