Finally, I managed to repair my Dreambox One based on the description of Myshel , the only change was that I did not manage to write the SD-card with the Libreelec Creator, therefore I used https://www.balena.io/etcher/ and with this I could write it immediately.
And yes,after installing GP4.1 you should use the Recovery Mode only and start installing the final image.
I am using actually this Merlin image and it is working fine: https://feed.dreamboxtools.de/oe_2.6/deb/images/