I solved forcing the dns resolution of the name fritz.box in the file hosts of the openhab server
In linux you must edit this file: /etc/hosts
If you are on windows the file is here: C:\Windows\System32\drivers\etc\hosts
You must add this line :
YOUR.ROUTER.IP fritz.box
So for example if your fritzbox has IP 192.168.1.1 the line should look like this:
192.168.1.1 fritz.box
With this modification your openhab server can resolve the name fritz.box to the specified ip.
edit: In my opinion it’s a bug…because if you don’t use the router as DNS you cannot resolve that name…and I don’t see why the binding is asking for the router IP and then ignores it trying to resolve it by its own.