Apparently the cable to the screen is to blame and the workaround is a piece of tape to block UV light reaching that component: https://www.reddit.com/r/xteinkereader/comments/1qlyq3s/this...

Personally, I've found that just moving the screen out of the sun (for example by turning it upside-down, or holding my hand in front of it) when refreshing a page is enough to prevent it.