After reading some blogs about forcing the NoDo Update for Windows Phone 7, I had today the idea to deactivate the data connection and wlan at my HTC Mozart. After I have done this and I connected my phone via USB to my notebook – Surprise, surprise: The message for the NoDo Update appears.
After this I activated the celluar data connection again and everything goes his normal way. After 15 minutes with backup and installation I had the NoDo update on my phone.