I get kicked from my friends realm after about 10 seconds

I'm playing on version 1.15.2 (Java) on my pc. I have windows 10 and my computer's interior is not the problem.

My friend made a realm for him, I and on other person to play on. They can join and play but I can't. It doesn't matter who joins at what time.

What happens is that I join the server, it joins and I can play like any other person for about 10 seconds. Then I freeze for my friends but I can still move around on my computer. I can't open stuff, all entities stop moving, I can break blocks but they don't drop anything and only I can see them as broken (they reappear after i rejoin) and i can't pickup items. I then get kicked and the screen says "Internal Exception: java.io.IOException: An existing connection was forcefully interrupted by an external host" (this is translated from Danish so it may have a few mistakes)

This only happens on vanilla (addons also) but if it's changed to for example a minigame, I can suddenly join and play.

I have tried restarting my router and pc and playing with wifi instead of cable. Nothing worked.