Well, because the error message ran through so quickly I decided to video it - I should NOT have to do that PreSonus!!!
It said "Restart requested by audio device" - so I did a system restart, but that made no difference. From the subsequent error messages I guessed that, after setting up the MIDI device, StudioOne attempted to default to Windows audio. For some reason, this seems to put enormous load on StudioOne every second or so, and this was what causes the clicks.
So I set the configuration to "no audio device" and restarted StudioOne. This time it started without an error, but did not respect the "no audio device". Instead, it initialised ASIO4ALL v2 and seemed to work - except there was no audio (even though ASIO is configured to play through my speakers).
So I dug out my AudioBox, and it worked.
Sadly seems to be another example where PreSonus struggles to handle programming reliably at the system level.
I have a problem with NOTION since it won't work with my graphics tablet, using its 2017 driver set to absolute positioning ... but both the tablet manufacturer and Microsoft have confirmed exactly what the problem is and that it can ONLY be a programming error which should be trivial to fix - but they show no interest, using the excuse that they don't support legacy devices (unlike Adobe Audition.Photoshop etc, Corel, Sony, Microsoft, Sibelius, MuseScore, ProTools etc, which all work exactly as expected).
I don't know if that helps you John, but it does seem to be a problem with PreSonus and the system.