I THINK I can clear Calibre as a "suspect" for this problem.
I didn't do any upgrades, so what did I do to trigger this ?
(calibre doing almost full opf backup of main library)
Did some convertion tries of a book, couldn't get it right, SO remove the mobi from Calibre, removed the same from collection on K3, and deleted same from K3.
Bought same from AMZ, sent to K4PC, imported into Calibre(ssssshhhh), sent to K3, everything ok, "I think, so far".
Forgot to check libr. opf back status, naturally :-(
Then I did an import of K3 collections to Calibre using "the plugin" and boom
1688 books needed "opf back".
MEME: I'm NOT saying the plugins is the culprit !!!!!!! So don't worry !, yet
I have a blinding headache, but I hope I'll have the chance to test this all over again, before doing any upgrades of Calibre.
Don't get your knickers twisted yet !
This was just an update.
If this should trigger any ideas/things to look for during tested, I would be pleased, though.
Have a nice weekend,
(need to sleep, hopefully get rid of this headache)
P.S. Having colleeges: They instantly sympathized with me and said they were sorry I had "Phantom pains"
Ah well, I would have said the same