Thanks. I'm really puzzled. I did as BetterRed suggested and started in Safe Mode, and Calibre worked fine. So clearly it's something on my machine.
However, while I realise the anti-virus is the most likely candidate, I've just tried disabling my AV again and also MalwareBytes, and it still won't work. Plus I've been been using both Emisoft and MalwareBytes for a long time, and Calibre has always worked well before.
I guess I will have to work through applications methodically until I track down what's stopping it from working.
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