[DiapDealer]

My comments about Antivirus were more for @Vanguard3000's report of Preview not refreshing at all. Though I still recommend permanently exempting Sigil's program and preferences directories from Microsoft's Defender, as well as any other additional Antivirus software installed.

I also don't recommend having multiple, competing live scanning antivirus engines (a la Defender AND Kapersky) running at the same time on Windows. I've run into nothing but problems with that scenario. I find the built-in Defender to be quite sufficient. But that's a different topic.

EDIT: and before I forget, we need to verify that Sigil is not being launched via calibre's "Open With" plugin in this scenario.