QHYCCD have now replied to my support request, very professionally and helpfully, which is encouraging. I'm told their app works with Ubuntu 20. Meanwhile, I have discovered the software recognises the camera when the USB keyboard of my NUC computer is disconnected! So, the problem seems to be is Bus-related. I know interrupt conflicts cause problems, so I expect it can be resolved. I still have no idea why their app doesn't recognise the camera when it is connected to the notebook computer, though. Plan-A was to use it for polar alignment. It looks like their forum will be back up soon, when their web server has been updated.
You might take a look at the USB bus structure with 'lsusb -t' and see how the devices are related. Sounds to me like a bug between USB 3.0 and 2.0 devices on the same hub. Maybe try a different model of keyboard, and different position for the devices in the various ports.
Thanks. In fact it turns out that the driver was missing. Whilst I had installed .deb file, what I didn't realize is that I should have also downloaded the SDK from QHYCCD's website, as it includes the driver! I expected the driver to be included in the .deb file. When I saw SDK on their downloads page, I thought it was intended for developers (ie Software Development Kit). The SDK includes a file called install.sh which has to be executed, to install the driver. If anyone needs information on how to do this then let me know and I will gladly do what I can to help them. So, I now have the PoleMaster working with Linux Kubuntu 20.04 on my NUC and Kubuntu 20.04 on my Toshiba notebook. The SDK file is called sdk_linux64_21.03.13.tgz so it looks like it was last updated on 13th March. I have found support from QHYCCD to be excellent and would like to thank them for their time, patience and help. Now I just need clear skies to test it with starlight.