Approve / reject via email notification not working

Summary

We cannot approve various tasks using the links in the email notification

Content

We have recently had a 'Live' to 'Test' environment refresh. It has become apparent that when using the approve / reject links in the email notification, the desired action is not working. The email populates as expected and when it is sent we get no error messages. The tasks remain in a pending approval state until the task is approved in another way (BPM worklist etc).

We always have to switch our email notifications back on when this refresh takes place - which I have done (ADMIN tab within BPM Worklist > change notification mode to ALL - this is how we have if in our live environment). I have checked a related task - e.g. ReqApproval but the 'Make Notifications Actionable' seems to be checked.