Because it hasn't been said yet, have you rebooted your computer and tried again?
I'd suggest submitting a bug report with the epub attached. Find out
how to submit a report here.
If the ePub is copyright mark the ticket as Private. After creating a bug ticket you can mark the ticket as private by clicking on the Yellow icon with the exclamation point in it following the "This report is public" in the top right hand side of the ticket.