[DiapDealer]

No. The nav manifest property seems to be correctly set. The only odd thing I'm seeing (utterly ridonkulous gutenberg file-names aside) is that the nav document itself is created in the root of the archive (with the mimetype) instead of being alongside the opf file. Still, the href to the nav seems to be correct. There's some attribute errors in one of the html files (and warnings about the html extensions, of course), but other than that, EpubCheck seems to be content with the structure of the epub itself. Unless I'm missing something, I don't think Sigil should be throwing up that error. Especially considering that Sigil succesfully updates the href to it.

I'll attach the problematic version so you can join in.