My page doesn't validate

Dear Sir/Madam,

Suddenly, my page stops to be validated. It always used to be validated, but for some kind of reason it is not anymore.
However, this doesn't seem to be caused by used invalid HTML.

The first error has the following description:
"Error: could not get "/_library/dtd/tbm-html4.dtd" from "213.51.81.80" (reason given was "No acceptable objects were found")"

It seems that the validator can't get the DTD on my webserver. But I can reach it when I type the URI http://213.51.81.80/_library/dtd/tbm-html4.dtd in my browser. 

I have double-checked everything. Can you please help me?

You can reach my website by using the following links:
http://www.timvandermolen.yi.org/
http://213.51.81.80/

Thank you very much in advance.

Best regards,
Tim van der Molen