I have a site which has been running for some time. Recently whenever anyone makes a contribution (via PayPal). When they click return to merchant they get the following error:
This is caused by the qfKey coming back in the URL being invalid. The error starts on line 308 of civicrm/CRM/Core/Controller.php and is caused by CRM_Core_Key::validate($key, $name, $addSequence) returning null.
I have disabling all my custom templates and the issue persists. Looking at the code it should get the qfKey from the request, so what is causing it to be invalid?