Finally I`m able to use linux-rt with the desired resolution again. The
solution was to first remove any fglrx-driver. Then I fully deinstalled
the linux kernels and xserver-xorg and then reinstalled those. Why
didn`t I try that before?? Conlusion: I`ll never touch that fglrx driver
again!! I guess either of the installation or deinstallation of the
fglrx driver didn`t work properly and damaged some file or another. At
least now I know that the fglrx driver definitetly doesn`t work with
linux-rt. Anyway, thanks to everyone for your help!!
Cheers
Matthias