I/O Error when attempting CSS validation

When I attempt to validate the CSS from any page in my university's 
domain (truman.edu) I get an I/O error.  For example,
the URL:

http://jigsaw.w3.org/css-validator/validator?warning=1&profile=css2&uri=http://vh216602.truman.edu/agarvey/cs315/simple.html

produces an error containing:

CSS
Validator
Error


    Target: http://vh216602.truman.edu/agarvey/cs315/simple.html

I/O Error: vh216602.truman.edu

------------------------------------------------------------------------


However, when I replace vh216602.truman.edu in the uri in the query with 
the IP address of the machine (150.243.160.100) it validates just fine.

In the first case above my Apache access log shows no requests 
whatsoever coming from the w3.org site.

Note that html validation (using validator.w3.org) works just fine.

Can anyone help me figure this out?

Thanks,
Alan Garvey
agarvey@truman.edu