There was a regression in Sigil 0.3.3. This release is meant to address that. Changelog follow:
- fixed a regression ("Not a folder" error) with opening certain epub files on Mac and Linux systems (issue #731)
This was caused by a bug in the ZipArchive library which I updated from 4.0.1 to 4.1.0 for Sigil 0.3.3. The bug has now been fixed.
The bug only affected Mac and Linux users, so Windows users don’t really have a reason to update.