[enuddleyarbl]

I'm just adding a comment for what I'm currently doing so I don't forget it later (in case it's relevant). I finally got fed up with Windows 11 and went back (clean install) to Windows 10. My first editing session in Calibre went fine. But, Windows Security just let me know that Reputation-Based Protection (Windows Security > App & Browser Control) wasn't on. I turned it on and my very next session with the Calibre editor refused to open. I've now turned off Potentially Unwanted App Blocking (Block Apps, not Block Downloads) in that area and will see how things go. The Calibre Editor started up just fine this time. But, since it's a sporadic problem, that really doesn't mean all that much.

EDIT: Nope. Turning off Potentially Unwanted App Blocking (Block Apps) didn't work. Calibre Editor wouldn't launch on the second book I tried editing. Looking at the other things on that Reputation-Based Protection page, they all look like they're involving downloads and not apps installed on the computer. I don't know what else to try turning off. I'll have to look things over some more.