Yes, sometimes the text would only read "Betriebsbereit", sometimes it would go as far as "i = 60".
A few minutes ago, I set the baud rate to 115200 and removed most of the strings. It worked. The text went as far as
99
100
Versuch abgeschlossen.
Erfolgreiche Schaltvorgaenge: 100"
So it seems, that the length of the "i = " string was the problem.
Thank you for your help. I will now try using the F-macro.