[DiapDealer]

Sigil kept valiantly trying to correct all of my various links when trying to recreate the issue, but I finally managed to get the exact same error by moving the stylesheet file to the root of the epub and manually changing the embedded font src url path in it to "../Fonts/blah".

If you manually move a css file to the root of the epub container, Sigil will correctly adjust the font urls (if they're correct to begin with) to OEBPS/Fonts/blah (Note the missing "../" in the url). But if you manually add the "../" to the url and re-run epubcheck, you'll get the error. As it should be.