I'm sure you've all been following my travails with baited breath,
wondering what the root cause of my problem was. Well...
I finally resorted to rebooting MacOS. Now it works!
So, I have no idea what the problem was.
Oh, and the first time I ran quicklook after rebooting, it had two
failures, but completely different than the previous failures.
I ran it again and there were no failures. There's still some
timing issues or other bugs in quicklook...
Anyway, back to work now...