“We’re sorry, you are not allowed to proceed”

Hi there. First of all, the plugin is amazing! I am using it on every clients’ website.

But, on this particular website, there is a “booking” kind of option, with integrated card payment (local Bank’s system). After the visitor/buyer proceeds with checkout, he is redirected to external URL (Bank’s), and after successful payment, he is redirected back to new page “www.mydomain.com/successful”. But there is the notice
“We’re sorry, you are not allowed to proceed”

Can you please help avoiding the notice/error?

The page I need help with: [log in to see the link]