Indeed, @olivierlambert, that was the issue and it works now. So for whoever has the same issue. First go to the local IP address of the host (NOT to the domain name), accept the certificate and then go to /xolite.html and it will work both with the ip address and the domain name in the address field.
Thanx again for clearing things up!

@poddingue Fistst of all I appreciate your answer and your position.
The thing is that, even though the proxy code itself is opensource, the functionality of the plugin is basicaly behind a paywall. We are not talking about support. Actual functioning of the plugin after compiling from sources depends on license availability and there is no option to select no support or something along the lines "I built it myself from sources". Without patching the code even though the proxy is otherwise functional the backups won't work because of missing license. Hopefully the powers that can will provide an acceptable albeit community supported way to use the proxy cleanly, without touching license checks. Best regards!