Web reader fails with 404/500 on `~readium/guided-navigation.json` when the saved position is in a book's front illustration pages

With a light novel that has 8 pages of images between the TOC and the start of the text, if I open one of those image pages in the mobile app and then exit, opening the web reader for the same book and user fails to load:

http GET request for https://<host>/api/v2/books/<id>/read/~readium/guided-navigation.json?ref=OPS%2Fsection-0009.html failed with HTTP status code 404

Server log: ERROR Resource not found in book: ~readium/guided-navigation.json - Resource not found in book with "errorType": "file_not_found_in_book", ReadiumServiceError. Reproducible across several light novels from multiple publishers on 3.0.0-beta.14; sometimes reported as a 500 rather than a 404.

Resolution: The web reader asks for guided navigation (the readaloud sync data) for whatever resource your saved position points at. Front-matter illustration pages have no readaloud segment, and in some EPUBs the manifest declares a section that doesn't exist in the container at all, so the lookup fails with file_not_found_in_book and the reader refuses to open. Note the position isn't even saved while you page through the front illustrations, which is the tell. Workaround: in the mobile app, move your position to the first real chapter (e.g. the Foreword) and exit - the web reader then opens normally, and once loaded it can navigate back into the illustration pages without error. If your error names a section index higher than anything present in the EPUB, check the OPF in Calibre for a manifest entry with no matching file; a malformed EPUB reproduces the same failure.