Thank you to everyone who offered your thoughts on this issue. The problem turned out to be.......

Acronis True Image Smart Protection :-(

Acronis is a backup solution I've been running for a couple of years, so I have no idea why it chose now to become so aggressive that it literally made Poser unusable. Smart Protection makes backups and locks files against ransomware attacks, but it just suddenly crippled Poser. I'm used to anti-virus programs crippling your start up with boot sector scanning (it's why I stopped using ESET in particular), but for a backup to become so aggressive is quite incredible.

Once again, many thanks for all the suggestions, and your kindness in addressing my problems. You gave me lots of things to eliminate.