There are some interesting things to try here:
https://contentbase.com/blog/calibre...k-not-opening/
Down near the bottom, he summarizes things with:
Quote:
7. Quick Fix Checklist
- Confirm that the selected record contains EPUB, AZW3, or KEPUB.
- If prompted, choose the correct editable format rather than PDF, MOBI, or DOCX.
- Convert an authorized unsupported source to EPUB or AZW3 before editing.
- Test Edit book with a small, known-good EPUB.
- Check whether the editor is open off-screen or on a disconnected monitor.
- Confirm that the library file and folder are writable and available locally.
- Pause cloud synchronization only long enough to perform a controlled local test.
- Review antivirus history instead of disabling security globally.
- Start calibre with calibre --ignore-plugins to test custom plugin conflicts.
- Use calibre-debug -g or calibre-debug --edit-book for focused diagnostics.
- Create a temporary library before reinstalling or altering the original library.
- Stop troubleshooting as soon as the editor opens and saves normally.
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His instructions above that seem pretty detailed.