Hi @Kurt_Mutschlechner ----- Thank you for reaching out and sharing the observations made during your troubleshooting of this behavior.
In regard to your question:
“Can you tell us if there are necessary network requirements, so that we can contact our ISP.”
We do not have specific network requirements to be able to run Roon and while connection speed does play factor, this more so has to do with what gear you are using in terms of networking hardware. Our CTO Brian, wrote up a very informative post about what an ideal network configuration could resemble and what devices to try and avoid.
-Eric