Thanks Pipeline - yes, Cubase was hogging the audio device; changing the settings to release ASIO when Cubase is in the background fixed the problem.

fiddler2007 - The reason that I have Studio One is that at one point I had given up on Cubase because (for me) it was too flakey and I couldn't rely on it. I came back to it with the 9.5 release which (for me) was very stable and, so far, I have found Cubase 10 to be OK.