Originally posted by Fringe:

Thank you for your answers. I do have a AVM Fritz!Box 6591 Cable and would try that.
My ISP is Vodafone Cable (germany).

How would NAT help in this case? My router can't just make up any IPv4 for the server, which would be accessible from outside, can it?

It would not help. You are absolutely correct: Your friends will have no way to connect from outside since Valheim doesn't accept an IPv6 address as input. There are some services that provide an IPv4 to IPv6 bridge (I'm using feste-ip.net for that), but unfortunately, this only works with TCP connections, not UDP.

I've spend some more hours finding a solution but no success so far. Turns out, it's not even Valheim that's the problem here but the server browser built into Steam (yes, the interface you already know from CS 1.6) doesn't seem to have support for IPv6.