So ok, here is the next little update:
Conclusion from my discussion with the sony support is, that they don't want to do a remote diagnosis. Well, thats understandable. They told me to send it out to their repair center in texas and gave me the hint to do this via a friend or relative with an valid physical US adress otherwise the reshipping will be without any insurance. All in all the support was not that bad as i thought it would be. I got quick and friendly responses from them.
So i disassembled the device according to
igorsk disassembly guide. I cleaned the reader internally and checked all cables as well as their connectors. I looked at the e-ink when i had it outside the case and indeed it doesn't seem to be broken.
After reassembly the reader acts as bad as before.
Next i will send it out to sonys repair center and see what they will say to it. Hopefully it will not be a wast of shipping costs...