Does anyone have experience with editing epub formatted ebooks?

We are experiencing odd formatting errors in two ebooks. They were formatted and compiled in Calibre, and look quite fine there. However, when viewed in Apple Books, there is an odd error message:

“This page contains the following errors:

error on line x at column yyyyy: Opening and ending tag mismatch: div line 0 and p
Below is a rendering of the page up to the first error.”

(the 'x' and 'yyyy' are actual line/column numbers. They differ for each book since the location obviously differs in each.)

We don't know what this means, and can't figure out a way to fix it. One book is reported to be corrupted; although Apple Books opens it briefly, long enough for me to see the error message, I can't proceed  from there.

There is a challenge with epub that you gotta try to make it work with a lot of shitty epub readers.

In the calibre editor, there is a "verify book" button that should take care of it. (Also under tools)

If that does not work, can you copy the contents of that line here?