iTunes Producer ePub errors

I’m getting a bunch of errors, but I’ll try to keep them as separate posts/questions.

ERROR ITMS-9000: "No OPF XML" at Book/BookAssets

Any help is greatly appreciated.

For the curious, this is the reply I got from Apple:

"You encountered this upload error because one of the links in your book is missing text, refers to a reference that doesn’t exist, or is not formatted correctly.

“To resolve, check all the links in your book to ensure they’re formatted properly.”

As I investigated, it turns out that error wasn’t accurate. this error was caused by the invalid html (in this other thread):
viewtopic.php?f=2&t=54151

Once I corrected that (by hand) the error went away.