What version of the Kepub Output plugin are you using? There have been a few changes over the last year that could explain the difference between converting a book a year ago and now. The last error is in code looking for the namespace in the html tag. If that is missing, it is book has errors that the calibre editor would show and can fix.
The other error looks to be related to obfuscated font in the book. That probably means someone has removed the key but not the font. Again, it would be an error, but, I'm not sure if it is one that the editor shows. But, in this case, the error is one that would occur with a epub-to-epub conversion as well.
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