Hello Andreas,
Thank you for your message.
The issue might be caused by some issues with the DNS resolution, for example, it might occur if some free/public DNS service is used.
Please try the following:
1. On the Viewer/remote Host machine (depends on which module cannot connect to the RU Server) launch the Run dialog by press
Win + R. Type
cmd and click
OK (or hit Enter). The Command Prompt will open.
In the Command Prompt type in
nslookup and the DNS which is used to point to the IP address and see if it resolves properly:

In case if it does resolve properly, you should be able to see the IP address the DNS is pointing to in the nslookup output.
2. Please try using your external IP address as the server address instead of the DNS name and see if it's possible to connect to the RU Server this way. Namely, you need to specify the server address in the
Host settings,
Connection properties on the Viewer's side and in the
Viewer's Server manager as well (if you use RU Server's sync and authorization roles).
Please let me know if the issue persists.