The most common reason for receiving an error while identifying ePub slice levels during the slicing process is due to an invalid tag in the ePub file. Here are some of the most common:
The file references graphics files which are outside of the epub package: When the slicer tries to make a new chapter file, it needs to find all the referenced images in the chapter and copy them over into the new chapter file. If it can't find one it fails, and that will now cause the whole book splitting process to fail even if its just 1 chapter that has such a bad link.
Date value is invalid: The date must be in the form YYYY, YYYY-MM or YYYY-MM-DD (e.g., "1993", "1993-05", or "1993-05-01"). For example:
- Invalid date value <dc:date opf:event="original-publication">-380</dc:date>
- Valid date value <dc:date opf:event="ops-publication">2009-08-11</dc:date>
However, if you'd like to discover pretty quickly what the structure problem might be, we recommend validating your ePub file. More information about validating ePub files can be found here. Or go directly to the IDPF Validator tool to validate your ePub file.