Amazon UK recently introduced Smile - this enables one to associate a nominated charity with the account, such that Amazon makes a small payment to the charity when one makes a purchase.
As a result, when running PlayOn to access Prime Video, the Watchlist and Library are empty.
There is no way to configure PlayOn to use this URL. This needs to be fixed. I will raise a support case, but I thought I would post here to see if other UK users have experienced the same problem.