Originally posted by Fringe:

I was just about to try portmapping via feste-ip.net. It's unfortunate that even this way won't work. You saved me some time by letting me know, thank you!
Well then... I have to give up for now I guess :(

Sorry! I think, I spent about 10-15 hours trying to get this running from a IPv6 network and I'm running out of ideas.

There's still one idea I've got: Setting up an openVPN server in the same docker network with the Valheim server. I don't know if this would work but the idea is to tunnel the traffic from IPv6 into IPv4 using the feste-ip.net TCP portmapper and then being able to send and receive UDP packages in the encapsulated network.