If you or a customer see the warning: Security check fail .
You need to check that you have enabled COMPLUS as a parameter to be sent back (We use this to send some data to and from the ePDQ system).
Login to the ePDQ back office and navigate to here: Configuration > Technical information > Transaction feedback.
You will find the setting under the Dynamic e-Commerce parameters section (in blue)
Find COMPLUS in the available section and click the right arrow to enable it.