Found it: two out of three postprocessing options don't work - the first one, marked as „(built-in)...“ does. The slow one complains of libraw being the wrong version, the quick one of libtiff.
Just click at the cogwheel button (bottom left) and select the default postprocessing. That's it.
Just click at the cogwheel button (bottom left) and select the default postprocessing. That's it.