Solved: thank you for your feedback. The reason why I tried to install paperless-ngx was that I received errors during installation from the bigbearCasaOS repository and I found the advise on the net not to use it in this case but to use the app from Coolstore repository (whic also failed).
I finaly managed to setup up a now running paperless-ngx installation based on bigbearCasaOS repository but it took me several installation attempts (I always updated only the ip-section of the config with the ip of my machine). I still don’t know the cause why the previous installation attempts failed, but for now my problem is solved.