Unable to Determine Parse Mode and other related problems

One URL I test gives the "Unable to Determine Parse Mode!" warning as well
as the "Line 1
<http://validator.w3.org/check?uri=http%3A%2F%2Ftest1.calcxml.com%2Fcalculat
ors%2Fhome-affordability%3Fskn%3D502&charset=%28detect+automatically%29&doct
ype=Inline&ss=1&outline=1&group=0&No200=1&verbose=1&st=1&user-agent=W3C_Vali
dator%2F1.3+http%3A%2F%2Fvalidator.w3.org%2Fservices> , Column 1: end of
document in prolog" while a nearly identical URL (HTML nearly identical)
succeeds without warnings or errors.

Valid URL: http://test1.calcxml.com/calculators/home-affordability?skn=504
Invalid URL: http://test1.calcxml.com/calculators/home-affordability?skn=502

I understand that the problem would have to be related somehow to the slight
difference in the two htmls, but I have no idea what to do to find out what
the problem is. There is not enough information in the warnings and errors
for me to figure this out.

Questions:
1.	Why is it unable to determine the parse mode?
2.	Why does it think the html is empty?

If you browse to the invalid URL in a browser (except IE10 I believe, which
is why I am trying to figure this out), you will see that the invalid URL
does render and indeed returns html.

TIA,
Brian