After saving to disk, or using kiwidude's Modify epub plugin with update metadata, I get the error 'The "role" attribute is not an allowed attribute of the <creator> element.' when validating (FC and epubcheck). This is the offending line in content.opf:
Code:
<dc:creator xmlns:ns1="http://www.idpf.org/2007/opf" ns1:file-as="Last, First" role="aut">Last, First</dc:creator>
Prior to updating metadata the file had an empty creator tag. Changing the offending attribute to ns1:role="aut" (specifying namespace?) fixes the problem.
In a fresh install of calibre portable I observe the same error when saving to disk. Am I doing something strange, or did I stumble upon a calibre bug?
The attached file will reproduce the problem (for me) if added to an empty library and then saved to disk.