I'm still testing, but it appears you nailed it.
I've used Avast for many years without issue and then several weeks ago I started experiencing a much slower PC and several unexplained bugs in various software, including Calibre.
I uninstalled Avast and even without a restart, all the bugs disappeared. Calibre now running flawlessly again.
Now to choose a different AV.
I am open to suggestions. Anyone?
Thank you, Kovid. Not only for your assistance with this but for such a high "caliber" software. Pun intended. 