On my website on front page only i get this error:
XML Parsing Error: not well-formed Location: Row number: 1, column 153.
I'm running it on Wordpress if this makes any difference, there are no ajax calls submitted by me, maybe WP does it on itself however.
The problem wouldn't be serious if this wouldn't prevent my <a> links from working here and there, but it does...
Also the location of error is not really defined and i can't manage to find it.
Any ideas on how to debug it or fix it?
That error means that the XML is fundamentally unsound; it doesn't follow the rules for being XML. It might be missing a closing tag, or an opening tag might be malformed, or any number of other basic problems. For further details on what not well-formed means in XML, see this answer.
To debug this in the context of Wordpress, try disabling all extensions to see if the problem goes away. Then re-enable them one (or half of the set) at a time to narrow in on the culprit.