We tried again on the same network with a different computer and there was no problem.
So i really think that the internet is not the issue.

Well this issue was the exact same issue as here:
viewtopic.php?f=29&t=39988&p=237408&hil ... er#p237408

I don't think his Internet is the issue when we play overwatch together for muliple hours without any problems.

New bug report? Na I'll add it in here:
[14.22] (GOG Version)

Issue: One friend could connect to the server, (port-forawd not headless) while another keep dropping out.
After trying a few times, we managed to get a 30 min (or so) session without problems.

While trying to find out what the issue ...