Yesterday I could convert a PDF file to MOBI in Calibre - today I can't!  The PDF book likes fine on the Calibre viewer but won't convert.  I've downloaded the latest Calibre version and rebooted.  Nothing to do with Kindle for PC as I didn't get the book from Amazon.  Strangely enough I had yet another Windows 10 update foisted on me yesterday - could that be the problem??  Any bright ideas welcomed!