The minimal example was not representing the issue I had.
- the first comment was right. Without using of ptr_global, the example is useless for debugging. When using ptr_global, the value will be set. The issue was identical, so I expected the example to be representative. Somehow it was, but this is hard to explain.
- Nevertheless I was confused about accessing pointers and values in "data"- and "prog"-memory. I mixed it up unintentionally. Now I use the functions of "avr/pgmspace.h" for access.