The version of Ultimate SEO URLs downloadable from here is not compliant with PHP 5.3 nor with 1.3.9+ versions of Zen Cart.

And in certain circumstances (if you use cPanel as your hosting control panel) then there are problems with that. The cPanel cure is to update the Ultimate SEO URLs .htaccess file like so:

Code:
# All other pages
# Don't rewrite real files or directories
RewriteCond %{REQUEST_FILENAME} !-f [NC] 
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*).html$ index\.php?main_page=$1&%{QUERY_STRING} [L]
change to:

Code:
# All other pages
# Don't rewrite real files or directories
RewriteCond %{REQUEST_FILENAME} !\.shtml$
RewriteCond %{REQUEST_FILENAME} !-f [NC] 
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*).html$ index\.php?main_page=$1&%{QUERY_STRING} [L]
Vger