browser can see page but validator gets an error

hi,

i try to check an website but the validator gets an error all the time.

 I got the following unexpected response when trying to retrieve
<http://www.rocnet.de/>:

    500 Internal Server Error

Please make sure you have entered the URI correctly.

if i call the website with a browser i don't get any error, any ideas why?

regards

claus rosenberger