@jkandasa ok, its working well now. The root cause seemd to be that I not only uploaded the bootloader using usbasp but also the firmware sketch with the result, that another non ota bootloader was installed by usbasp, too.
Now I used a brand new barebone atmega328p in a really basic circuit with a NRF24L01+, upload the bootloader using usbasp only and then wait for MyController to recognize the node. Now I can upload the firmware by just rebooting.
Uploading the TimeReporter demo lasts ca. 6 minutes with a distance between gateway and node of about 6 meter in different floors.
Thanks for doing such a great job building MyController