Recently we had client experiencing an ugly error when adding a list item, which the error appears to be System.InvalidOperationException: There is an error in XML document (1, 43). Done lots of checks, but just couldn’t figure out why this happened until we found out it’s due to Internet Explorer 9!
This happens when you use Internet Explorer 9 on Sharepoint list with a People Picker field. It will throw the error when you submit.
See the following articles: