Probably the text files in the epub are too big (should be max around 260 kb each), I don't think Sigils validation checks for this.
Also, you can try this for more errorchecking:
http://www.pagina-online.de/software/epub-checker/
- it's an offline java GUI for the "official" epub-check tools, and partly checks for some other things than Sigil.
Regards,
Kim