Hey Marten.Lehmann,
I’ve tested this by following the guide linked below:
Using that guide, I was able to successfully build and join a dedicated server with a client. I’d recommend following along with that guide and then seeing if you get the same issue on your end. Also, it shouldn’t be necessary to include the :7777 as that is the default port that we will use as far as I’m aware. I did not include this on my end and it worked just fine.
Let me know what results you see after working through that guide.
Have a great day