It looks like your IdP has received an authentication request that’s not valid XML. Difficult to tell just from the stack trace, without any other logs or explanations about what you were doing. My suggestion is to try again with firefox with the SAML tracer extension installed and running. Then grab the SAMLRequest parameter, base64-decode it, and see how it looks like.
UNINETT / Feide
"Two roads diverged in a wood, and I, I took the one less traveled by, and that has made all the difference."
- Robert Frost