fyi, if you search by the title you are looking for both 4k and non 4k versions will show up. I'm experiencing this too, but amazon rep says they are on it, so I'm watching non 4k versions like a frickin' plebian in the meantime. :(
Can confirm this is still happening as of 1am on Sept. 12th, NYC region. 4k titles won't play, HD titles seem to be fine. All other streaming sources that I watch are fine. Have restarted and de-powered/re-powered with no solution. Not about to continue watching Carnival Row or any premium content in 1080. FIX THIS PLEASE!
I can confirm that turning off HDR from Apple TV Video settings fixes the issue and allows the 4K version of Prime Video content to play. It was working a few days ago just fine with HDR turned on, so I’m not sure if it’s tvOS 12.4.1 related or a change on Amazon’s side. I also called tech support. This thread was more helpful than the representative.
Same situation. Was working no problem day before yesterday. Put in a call to support, colossal waste of time. 4K works on every other service; Netflix, Hulu, and apps for PBS, iTunes movies and TV shows, everything. It's just the Prime 4K that is blowing out the ever-useful error "Something has happened. Please try again later." Not even an error code. Amazon has surely screwed something up on their end. Will they fix it? I'm not feeling encouraged by their ridiculously poor support staff nor Amazon's tech staff who clearly let the bull out of the barn on this one.
Given that this has been going on since 2017, am I to assume that you have no intention of fixing? All I see are excuses that your technical team is being contacted. Do they even exist? Just tell us the truth so that we don’t waste time. There are other streaming services that we can pay for that actually work.
Similar to other posts, 4K was working for us on Apple TV 4K two days ago and then suddenly stopped. We've tried rebooting, unregistering, uninstalling/re-installing the app - nothing has worked. We also have a non-Apple TV 4K [older version of Apple TV] in another room, and the 4K file will play on that old Apple TV [though not in actual 4K]. Our Apple TV 4K will play 4K HDR content from other apps, but won't play from Amazon Prime Video for some reason. This was working fine for the past few years, until just now.