TOPIC: 404 Error After Successful PayPal Transaction

We are getting a 404 error after a successful PayPal transaction has occurred.

We are using search engine friendly URLs on the site and the details article link works fine. However, the custom thank you page URL is not working and when we attempted to replace it with the hidden menu item's link (e.g. index.php?option=com_content&view=article&id=XXX) it still throws a 404 error.

We are not using any special component for our SEF on the site, just what comes built into Joomla.

When we next attempted to remove the custom thank you and have a custom message appear instead, we still get a 404 error. And the same result happens when we shut off the custom thank you message altogether.

We also updated from v3.1.6e to v3.1.6f and are still experiencing the same outcome.

Please advise ASAP.

Thank you,
Doreen