Sorry for the late answer, i was away for some days.
After the weekend i tried to configure the clients again to have a fresh RCS-Log.
Surprisingly it worked on two of my clients and they are now shown with the status configured in the SCS console. Perhaps the OTP Error really needed some time to resolve.
Now i only have to get SCCM 2012 to recognize the clients to manage them.
On the other clients i found that the unprovisioning under SCCM 2007 must have gone wrong and they are partly provisioned with an old certificate from a Server that dosen't exist anymore. The ACM_Unconfigure_SCCM Task isn't working either.
Do you probably know a way to reset the Clients without logging manually into the MEBX? I fear that there are some more around.