i have found the solution. :-) The problem was, that in the Windows Phone project there was a wrong version of Microsoft.Practices.ServiceLocation.dll. The MVVM-Light Package by default installed there the v2.0 version of the dll. In the MVVM Light PCL project by defaukt it was the v4.0 version. So i changed the reference in WP8-project to the v4.0 and now it's debugging.
So if someone else runs in the same problem as me, here is the solution.
Greetings, Daniel.