Hello, Paint.Net seems to corrupt images when saved as PNG in very rare cases. In fact it only occured once to me. Possibly because the image is relativly big (11 MB). Maybe it has something to do with "Layers". Sorry, I cannot say what I have done exactly. Maybe the image itself gives a clue when analyzed...
The OS is: Windows 8 x64 Pro
http://www.file-upload.net/download-7570794/bugreport.zip.html
Thanks for your help