I/O Error: Not Acceptable: bug?

Hi,

when I try to validate my website, I get this error:
   I/O Error:  http://ozy.student.utwente.nl/Home/: Not Acceptable
I cannot validate my pages, though AFAIK everything seems to be in 
order on my pages. Besides, the HTML validator does work, and validates 
my pages as correct XHTML. Perhaps this is because I use quite a 
special method for constructing the pages so that there is no file 
behind http://ozy.student.utwente.nl/Home/ (no /Home/index.html or 
other). But I don not see any error in the pages constructed, so the 
CSS validator should work. My guess is that this is bug in the 
validator. Probably the code that gets the style sheets from the HTML 
pages.

My pages are located
   http://ozy.student.utwente.nl/
and on the main page there are two links (bottom of page) to the 
validators (html and css) which use the referer.

-Ozy