I recently tried validating a compiled epub file on the idpf.org site. Any failure on this site would invalidate uploads to Apple etc. I found the fix. I hope this is useful to people.
The file failed with the following:
WARNING OPS/toc.ncx - - meta@dtb:uid content ‘e6f9dd53-a44e-4b62-91d1-3c5b7cae613f’ should conform to unique-identifier in content.opf: ‘urn:uuid:e6f9dd53-a44e-4b62-91d1-3c5b7cae613f’
After much searching I eventually found out that ePubChecker released a new version of their program (v3.0.1) which changed, slightly, what constitutes a ‘valid’ file. This means that .ePubs that previously verified fine no longer do such that Scrivener does not now generate valid pubs. The fix is somewhat complex but is described by David Adams on the kboards.com/index.php?topic=154646.0 ). The fix described the need to change the entry in the toc.ncx file from:To The actual numbers appearing will be different for each person, what is important is adding the urn:uuid: text.
David describes how to make those changes in Notepad on Windows, TextEdit would do it on the Mac. I used BBEdit to do it and can confirm it all works hunky dory.
Hopefully the next Scrivener update will fix this.