Validating the cited URL with the 0.8.0 online Markup Validator results in the following error message:

    Line 12345, Column 0: — .

Not only is the XHTML valid (as far as I can see), but this is only a 16 line document!  The key line is the following body text (line 12) within a <p> container:

    :12345:&mdash;

The line number reported is the number within the colons--you can change it and line number in the error report will change to the new value.  I have made the following observations based on experiments:

   1.  Changing the text within the colons to an alphabetic
       sequence eliminates the error message.
   2.  Changing the text entity from &mdash; to a numeric
       entity (for example &#8810) or an entity in the
       US-ASCII range (for example &quot;) eliminates the
       error message, but changing it to a character in the
       ISO-8859 0xA0-0xFF range (for example &eacute;) produces
       the error message.
   3.  Adding arbitrary alphanumeric text between the second
       colon and the text entity does not affect the error
       message, nor does text including markup (for example
       "<b>text</b>").
   4.  The error message is produced for the XHTML 1.0 Transitional
       DTD as well as the Strict.