Code:
The <item> element's "media-type" attribute has value "application/vnd.ms-opentype", but the file's media type is "application/x-font-ttf".
Sounds like even though you have no font-files included in the epub, there's a manifest entry for one (or more) in the OPF file (either that, or there IS a font file included in the epub and its media-type is simply declared wrong as indicated).
If the font files are truly not included in the epub, then you need to remove the <item /> entry for it in the manifest section of the OPF file (it would be removed automatically if the font files had been deleted from within Sigil).