Hi Borate, many thanks for your suggestions.
Well, hint # 1 didn't work alone, although I changed to MATCH MONITOR. In export I didn't find any possibility named A/R. But I followed your hint to switch to Letterbox for images/videos. Finally, when I exported the video I chose 1920 x 1080 - 1080p-HD. Before it was set on Customs. I cannot remember if this was set with all the many other videos I made before or if it was changed sometime accidently. But in the end - it worked ! The exported video is as it should be now. As always, small mistakes can have annoying effects.
Many thanks a