[F.W.]

Originally Posted by kovidgoyal

http://manual.calibre-ebook.com/faq....s-occasionally in particular the last point.

Thanks, Kovid, for the quick response. Indeed, SpiderOak (which I have installed a long time ago but never ever used) was the culprit. Why the freeze / crash of Calibre only occurred just now, I don't need to know. All that counts is that everything works nicely again!