Friend is kicked by the Hamachi server in Minecraft?

I would like to play along with a friend who lives a little further away, so I decided to create a server. The server is ready, all mods are on it and he has all the mods and can also join the server but is kicked after 30 seconds at the latest. The console will always say this [19:57:33] [Netty Server IO # 0 / ERROR] [FML]: NetworkDispatcher exceptionio.netty.handler.timeout.ReadTimeoutException: null
Who knows where the problem lies and how I can fix it?

Leave Hamachi away and you better free the port in your router… Then the problem will disappear…

My father does not want to do that and I do not have the necessary password

Then forget about hosting and let another host - or rent a server. Did not cost much…

(Unfortunately, you have an unsuspecting father, because if he knew what nonsense you can do with Hamachi, he would immediately release the port…

If that helps, I'm also at your dad's age, most likely)

Thanks, I'll see if I can host the server to one of my friends.