I'm trying to forward a port to my server. I already have a couple of ports forwarded since a couple of years, but when I now try to add one more port I get the error message "The IP-address is already in use. Select another one."

I type a name for the rule, select the server in the list of clients, select the internal and external port, and click "Save".

What does it mean "IP-address already in use"? Of course it's in use, it's used by the server!

I'm using the Android app BTW.

Any ideas why it's not working?