@KevinH: I may have led you astray a bit. I did a full rebuild (without the latest off-the-record change) and I'm no longer getting the "deadline expired" message. It's still not deleting the empty 'Preview- +uuid' folders from C:/Users/<USER>/AppData/Roaming/sigil-ebook/sigil/QtWebEngine after Sigil closes, but the infinite loop does NOT seem to be happening with your previous change.
Sorry for any potential confusion. Should I still try the new off-the-record approach or do you want to revert and do more testing?
I think the new QWebEngineProfileBuilder class in 6.9 is intended to fix this exact sort of issue:
https://codereview.qt-project.org/c/...ngine/+/597808