Solved it.... but it was not the windows update:
I first applied any other update I found, in the hope there is some superseeding update in the queue which fixes it again. But no.
I then remebered, I also installed "Windows Desktop Experience" on the server because I needed the media player features and dlls.
But the Desktop experience also installs the **** Windows Mail app - which in turn enables itself as the default mail application (but i is unable to handle mapi).

Now, you cant uninstall windows mail, unless you uninstall the desktop expierence, and there is no default application you can assign the mail protocol as an alternative availible.

open regedit:
under HLM\SOFTWARE\Wow6432Node\Clients\Mail
you find a String "Default", which is set to "Windows Mail"
changed that to "ExchangeMAPI"

and voila, without a reboot, my BES is running again.

Maybe this helps somone else: be careful before installing Desktop expierence on a Windows Server.