Okay...i got a bit further now. The problem with that "M7" error seemed to be an CWM issue.
So i tried to reflash stock recovery....but my fastboot on my windows machine is broken and i was not envy enough to fix that.
So i grabbed my UNIX based MAChine ( ) cause everything should work there. And it did. But after starting the update process i got an error-message on my phone which said something like "system/bin/app_process" has unexpected contents"