Barclays epdq issue

We run a site at 

The site uses CS Cart 1.3.5. We have been taking payments with both Paypal and Barclays epdq for some time with no issues. In the last few weeks we have had issues with Barclays epdq. When a payment is completed the screen does not go back to the shop instead we get a browser error page with these words:

Safari cannot open the page The error is: “cannot decode raw data” (NSURLErrorDomian:-1015)

The order is going through and the system generates 2 emails for these orders instead of one.

Barclays say the issue is not their end.

As a point of interest when I try to access the POST URL used in Barclays epdq (ie by posting it in a browser window) we get an access denied message. Is this correct?

if anyone has any thoughts I would be happy because my client is getting very frustrated.

I believe the issue is a CS cart issue. When I log in and click to view my order I get the same error message, even on old orders. A similar message in Firefox. Would anyone know why orders cannot be viewed by users? Happy to pay for help if you know cs cart 1.3.5