[DiapDealer]

If it's a malformed xhtml issue (as the translation of the message seems to indicate it is), the Well-Formed Check EPUB tool (either via the Tools menu, or the F7 shortcut) should give some specific locations to look.

If there's a lot, you'll need to fix them manually and rerun the F7 Check again, until there are no more well-formed errors left (if you don't want to take your chances with the automatic Mend).

In the future, try leaving the Preview Window open when making manual html edits. It will let you know there's an issue with what you typed immediately. That way, they won't snowball on you and leave you flabbergasted when you go to save.