SAME HAPPEN TO ME WHEN UPADTED TO PIE WAS WORKING UP TIL THEN-----YOU COULD TRY SIGNING OUT OF APP AND SIGN IN AGAIN IF NOT TRY CLEARING THE CACH FOR THE APP IF NOT AND UNINSTALLING APP AND REINSTALLING BUT THIS DID NOT WORK FOR ME HAD TO RING SUPPORT TO DEREGISTER MY PHONE RESET IT THEN IT WORKED STRAIGHT AWAY TELL THEM TO DO THIS AND SHOULD WORK FOR YOU ALSO BY
Have just spoken to support who wern't aware of the issue. They did however tell me that you can deregister the device yourself through the Customer Zone; within your account, under Vodafone TV, select Manage my vodafone tv devices and delete the smart phone that is not working. Re login to the app on your phone and it works!
I had also tried deleting cache and reinstalling the app - which didn't work.