XHTML validation error

Dear Colleagues,

When trying to validate

http://www.informatik.hu-berlin.de/Institut/struktur/systemanalyse/kimwitu++/fancy.html

I got an error saying that 

  Uh oh! I got the following unknown error:
  /usr/local/bin/nsgmls:http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd:79:5:E:
  "SYSTEM" is not allowed as a reserved name here Please make sure you
  specified the DOCTYPE properly!

It seems that the validator has a problem with the XHTML DTD, instead
of with my specific page. Since I took the reference to the DTD right
out of the spec, I feel it should be correct. 

Any comments appreciated,

Martin