Roon core and HQplayer is running on the same computer. HQplayer added using localhost and/or 127.0.0.1, neither would work. The network control of HQplayer is on and both firewalls from AV software and windows are off. And clues?
In my setup, with HQPlayer and Roon on the same machine, I used the actual IP address of the machine not 127.0.0.1.