Correct URI not accepted (http://www.w3.org)

Hm:

Validate documents by URI with
--
http://www.iam.unibe.ch/~schlpbch 

Gives me:

Target: http://www.iam.unibe.ch/~schlpbch 

org.xml.sax.SAXException: Please, fix your system identifier (URI) in the 
DOCTYPE rule. 

--
Target: http://www.w3.org

I/O Error: Unable to contact target server www.w3.org:80 after 3 tries. 
--

Please debug,

Andreas