Finally this shit seems to be over. I checked and cleaned all major cable connections without any effects, and two rounds with resetting the log etc. only had short time effect, but my problems with the DSC 4x4, Engine Failsafe etc. anyway seems to be solved now.
I took a fight with the local dealer and they finally accepted my problems being covered by BMW guarantee. They told me they had to replace a control unit, although I never got any accurate information about which of them.
Anyway I am quite pleased with my car at the moment, but I still have an unsolved dispute with the dealer because I now want my money back for their initial and useless attempts to solve this problem.