Payment Processing Issues
Merchant Gateway Declining or Failing
Take a look in Billing > Gateway Log to see exactly what response your payment gateway is returning and that should indicate what the problem is for you. It would then be necessary to look up the error codes in your payment gateway's documentation or contact their support team for an explanation.
The explanation of common errors are located in the Troubleshooting sections of each payment gateway's dedicated page of the documentation.
Blank Payment Gateway Page
This is usually caused by a double copy of a gateway file in the modules/gateway directory or .htaccess/php.ini or other file not provided with WHMCS in the directory that needs to be removed.
User Redirected to Wrong Page on Checkout
If you have customised your WHMCS header template and added or removed a form, you will need to update the forwardpage.tpl template file which by default will submit form 0 or 1 on the page - if you add a form to your header need to add 1 to the forms[x] setting in that file, if you remove one, you need to subtract 1
Invalid Access Attempt
To avoid this error, please ensure you are logging into your admin area using the exact URL defined in Setup > General Settings.
If for example your WHMCS System URL is set to http://www.example.com/whmcs/, you would need to login to your admin area at http://www.example.com/whmcs/admin. If you login at http://example.com/whmcs/admin this can cause the Invalid Access Attempt message (note the www. was omitted).