Curiouser and curiouser ...
Last week Capture One support asked me to add a new admin account and install and run C1 from there to try and narrow things down further. Which I did, with precisely the same error on that account.
This morning I had a further email requesting the C1 logs from the new account.
Prior to switching accounts to the new one, I first tried opening C1 again from my original account, and it showed the identical original error.
I then changed to the new account and tried opening C1 again from there before saving the logs, and this time it opened normally from that account.
Even more puzzling, after reverting back to my original main account C1 opened correctly in that account on the first attempt and now continues to do so every time without error. Just to be sure, I rebooted the computer and C1 continues to start up as normal on the main account.
I had made no changes to the system during the course of the above, which was all one continuous process with no interruptions or deviation. Nor, according to the Windows update history, had there been any recent background system updates apart from a MS Defender security update at some unspecified time this morning.
I am completely at a loss to explain the change in behaviour, unless the previous MS Defender security definitions had perhaps silently blocked an essential file which had then been unblocked in this morning's update, but that's possibly clutching at straws. Unfortunately the update history doesn't give an exact time for the update; the nearest I can get to that is that it states the system as being up to date at 10.23 a.m., which does roughly coincide with the times when I was working through the above, but I can't be sure where I was in the process at that time.
I've asked Capture One if they can perhaps shed any further light, otherwise I'll just have to put it down to one of those weird computer glitches that seem to happen from time to time.