Even the type of epub that triggered the bug in 0.9.16 (fixed with
https://github.com/Sigil-Ebook/Sigil...517017fbc659d5) doesn't stump 0.9.17 on Windows or Linux. I've tried turning off Mend on open to see if I can trick the error message into showing, but can't get it to happen.
If the bug reporter is truly using 0.9.17, then it has to be either a MacOS thing (either a bug or something with their specific system), or it's just a different scenario that triggers that same spurious error message in a way we haven't duplicated yet.