What to run
Open XML Formatter. It stays in the browser.
How to read the result
Formatted output means well-formed.
The error is the first well-formedness break: unclosed tag, bare &, or mismatched close.
The page does not validate against an XSD.
What to open next
Open HTML Escape if you are about to paste the XML into HTML. It stays in the browser.
Open Find and Replace for a repeated bad character, then re-parse. It stays in the browser.