I had that Amazon app issue on my HU8550. I reset Smart Hub and the problem hasn't returned. It was after an update by the way.
UN65F9000AFXZA + SEK2500U/ZA firmware version 1150.5
Denon AVR-2313ci, Denon AVR-S900w, Apple TV, Sony BDP-S6200
Cox Cisco 6 tuner Contour Box , PS4, Xbox One, HD Antenna
(2) Klipsch F-30s, (2) Klipsch F-20s, (2) Klipsch S-20s,
(2) Klipsch S-10s, (2) Klipsch B-20s, (1) Klipsch Sub-12HG
UN55HU8550, Ceton Echo, Apple TV, Sony BDP-S590, Motorola Cox Contour Receiver, PC