Reinstalled 3.20.
Opened an ePub, followed your instructions, and the following error is displayed under Console:
Failed to load resource: the server responded with a status of 404 (Not Found)
https://calibre-internal.invalid/ron...ges/cover.jpeg
Will not scroll through or display any pages.
Went back to 3.16, and displayed fine, no errors. Can try completely deleting Calibre and all config settings and reinstall, but don't want to go through the hassle of reconfiguring again and reinstalling the DeDRM tools etc...