Thank you very much for your prompt answer.
The hint with the log-file helped a lot. For some reason two of the Add on scenery have been ignored because they were "not a directory". Pure chance this where the "new" ones I added. This is why there was no AFCAD of this scenery detected in spite of GSX's rebuild.
Long story short: created a new fsx scenery directory and added it to scenery library, put all AFCAD into this, started FSX (rebuilding cache), started GSX (rebuilding cache) and finally deboarded the passengers
Thank you once again!