thanks Ilkka, you gave me the important hint.
I was not willing to make even with a small SD card of 4GB senseless pictures until it was full. But filling it before to a certain extend did not come to my mind.
Inserting it empty the camera showed 258 pics, prefilled I did my test with 20 pics left, which were indicated at the cameras display.
After taking those, the camera did not release anymore, even the main card was more or less empty with more than 5K pictures remaining.
Switching in the menu to: main card RAW and SD card reserve made the camera work again, showing the total amount of pics on CFE card
Now at least I know, that the camera react the same way, no matter which card is filled