Here is a previous thread that goes over some troubleshooting steps, where a similar problem was found (it starts with Mobi, but we end up on ePub anyway).
As noted there, you’ll have a lot more precision in troubleshooting if you load your .epub into a program like Sigil, which will not only enumerate errors (likely with the same utility that Kobo does), but it will guide you right to the spot where it happened. Even if you don’t understand the ebook markup, pasting a bit of what you find can help. These error messages are meant to guide you to problems, they very rarely fully explain the solution all by themselves, or even the full context of what went wrong.
I should add, it can also help to search for the error message on the forum as well, after removing any parts that look specific (like file names or line numbers).