Cached "Page not found" for anonymous users
Is the page actually handled by Drupal or is it a static .html file?
It looks like a file that shouldn't be handled by Drupal, the 500 error 
http://www.checkupdown.com/status/E500.html indicates a fatal server error.

If it is handled by Drupal can you change the url to one without the .html &amp; see if that works with caching. You could use the global_redirect module to forward the .html addresses, once you figure out why it's failing.

If it is a flat file you may need to change the .htaccess or settings.php file to allow the .html to be processed by the webserver without bootstrapping Drupal.

There is a massive thread about this issue on Drupal.org http://drupal.org/node/76824#comment-3299738 You may find an answer in there.

PS 
If this is served on a shared host you may want to contact your provider, they can have some strange setups &amp; can fix quickly if you explain clearly what is happening.