Error requesting DTD-file on my server

Dear Sir/Madam,

When the WWW-validator attempts to request a DTD-file on my server during
page validation, it receives a "400 Bad Request" from my server.

In the error log the server logs the following:
"[error] [client 18.29.1.50] Client sent malformed Host header"

Is this problem caused by a misconfiguration of my server or by the
WWW-validator?

My website can be found at http://www.timvandermolen.dyndns.org/ or at IP
213.51.81.80 (port 80).
I am running Apache 1.3.22 on Windows.

Thank you in advance for your help.

Regards,

Tim van der Molen