If you use friendly links, then try to edit file .htaccess and change .html$ to .html. If it wll not help, then check if mod_rewrite is turned on your server. If it is not working, please ask your admin to turn it on. If it is working, you can try in file .htaccess after "RewriteEngine On" put line:RewriteBase /
or if it doesn't help and shopping cart is in subdirectory, then you can try add:
RewriteBase /name_of_directory/

On some servers problem is caused by first line in .htaccess file:
Options +FollowSymLinks
You can try to remove it.

Unfortunately it works different at various servers, so there is no one solution.Maybe server has problems with sessions. Sometimes is enough to create directory with name "tmp" and give chmod 777 to it. If it will not help, please try this:
How to store sessions in cart directory?

Please check if the script is installed correctly.
How to install our scripts

If tips we wrote won't work, please send us data for ftp connection, so we may look at your server and find the reason.