CantOpenError: CantOpenError: unable to open database file
Has nothing to do with EPUB (or any other format)
Are you running with the Library on a Network share (or NAS)?
Well, it finally bit you.
The FAQ has had an entry against this for as long as I can remember.
https://manual.calibre-ebook.com/faq...rked-drive-nas
If multiple people need ACCESS to the Library, run the
content server on one PC and have the rest use their browser to get or read the books