[enuddleyarbl]

Back from my reboot. The process repeats. After booting, Calibre won't run (well, it does, but as described in my OP). Turn OFF Windows Security's Real-Time Protection and Calibre runs fine. Turn it back ON and Calibre still runs fine. Until a reboot. And yet, nothing in Windows Security gives any indication that it has any problem with Calibre.

I'm going to try reporting this to Microsoft. But, that's probably a worthless endeavor. I suppose I can try setting up an exclusion in Windows Security. But, I'm not sure what to exclude. The Calibre executable? Its whole folder in Programs Files? Perhaps its folder in AppData\Roaming?