I had a similar "fading letter" problem and it was definitely triggered by heat (i.e. in sunlight or near to a light bulb). I got a replacement unit for free, and this now works in the sun as well. There are indeed quite a few people on the German forum with the same issue. One thread (with a good picture) is here:
https://www.mobileread.com/forums/sho...d.php?t=168495