On the checkorder.php script, the link for a customer to go back to their order returns the following 404 error:
/payment/\"../users/\" was not found on this server.
A problem in 1.6.4 version of the of the script which added unnecessary backslashes to the strings in the language file (Lang/english.php)
1. Upgrade to the latest version of the script.
2. Open the Language Translation tool, find the string "../users/" and remove all back slashes from the string. Click any of the save buttons to save.