I asked a blind installer who is explicitly familiar with Storatext blinds.
He was able to give me the following instructions by phone, which also led to success:
1. Lower the blind to about the middle.
2. For the upper end position, press and hold the recessed button labeled "auf - up" with a thin screwdriver.
3. Now let the blind run upwards and keep holding the "auf - up" button for about 5 more seconds.
4. When the blind has reached the desired end position, let it run in the opposite direction for about 5 seconds. --> The upper end position is now programmed.
5. For the lower end position, lower the blind again to about the middle and now press and hold the other recessed button with the thin screwdriver.
6. Let the blind go down and hold the button pressed for about 5 seconds.
7. When the blind has reached the desired end position, let it run upwards again for 5 seconds. --> The lower end position is now also programmed.