"Software error" when validating an XHTML fragment

I get the following error when trying to validate an XHTML fragment 
(whose source I am not allowed to copy in here):


  Software error:

HTML::Template->new() : found <//TMPL_IF> with no matching <TMPL_IF> at /usr/local/validator/share/templates/en_US/ucn_output.tmpl : line 152. at /usr/share/perl5/HTML/Template.pm line 2389.

For help, please send mail to the webmaster ([no address given] 
<mailto:[no%20address%20given]>), giving this error message and the time 
and date of the error.