[Phil_C]

Here is the strange solution to my problem:

I moved my library to a different logical drive, then ran the calibre Welcome Wizard to point it to the new location. Interestingly, the Wizard gave an error about the folder not existing, and crashed --- but found the library on the next start. Everything worked right at that point.

I then repeated the entire process, moving the library back to its original location on H: (and got the same Wizard error). But once again calibre found the library on the next start.

Everything appears to be working fine. Very weird. But now I can go back to enjoying Kovid's program.

Oh, and I'm no longer running in compatibility mode.

Thanks for all the replies.