I contacted the provider already and they could not provide help.
I mentioned the Mac because I was sure it was not a network issue, since that connection was ok.
I setup a quick Kubuntu system and tried it there just to make sure. The deb packages did not install at first because of dependency issues, which was surprising since debian is a supported sytem.
Turns out that multiarch support was missing, apparently it was discontinued. I found some packages that fixed the dependencies, then the installation was possible.
So CodeMeter worked on Kubuntu, after a quick fix, license transferred. So it’s definitely not a network problem.
I’ll keep this up to date, since others may have similar issues