Green "View Invoice" button not working for customers

Several of our customers have informed me that when we send emails (or our automated reminder emails go out) they cannot view the invoice by clicking on the "view invoice" button. They can open the PDF but not view online.

Had a facetime call with a customer today and could see clearly the green view invoice button and when he hovered over it a link string but when he clicked on it nothing happened - did not even open a browser..?

Have looked through other posts and tried setting customers card country, 

logo has also not changed since we started with myob and of course this feature used to work ok

Any ideas?