Don't know if the following info could help the developers solving the issue: If I connect via VPN to my network from outside, the involving PCs (which are not capable of running the host, so that a client can cannot without the error message) cannot be called by their windows' hostnames! IP only.
After checking the whole network setup I can tell you, that this error was indeed caused by an error on my side. I just found out, that my DSL modem from one of the biggest providers in Germany does not give you an IPv4 address as long as you don't pay for it. Even the router's not capable of portforwarding - d'oh!
So thanks a lot, I will always recommend your wonderful Remote Administration Tool!