I'm also having problems with it. Only works intermittently. I get the black screen with the white line across the top. If I continually try, for varying lengths of time, it's not consistent, I can eventually get it to load. But most frustrating.
Since using amazon prime video on v6 box been inconsistently working. Does appear to be related to when you’ve used the app once (in my case exited out from a program). Once this happens and you try to use the app again, you get a black screen with a thin white line running horizontally along the screen.
Rebooting the box normally fixes the, but not always possible due to recordings.
Just a heads up for what worked for me. I was getting the blank screen/white line combo this evening for the first time. I'm using channel 228 to get to Prime. Looked through this thread and tried setting the video modes to all ticked, and then the Amazon home screen came up, but in low resolution. When I started playing Vikings though, the res was really good - better than I've had it before.
Home/Help & Settings/Settings/Video Output/ Video Output Format/ then tick the highest resolutions (1080 and up works for me), and then click continue.