I've been having this random problem where some AO3 fics that I've downloaded as epub open just fine in calibre but then give me a page error on my ereader (Sony 650). If I delete from reader and then run it through the convert books setting (from epub to epub) and reload on my reader, it works just fine. Have you come across this issue at all?

Eventually, I'll file a bug report with AO3 but if there is more data that I could give them, it would be helpful.