Dear Abhishek
Yes, now things are finally working. I can onboard my App again without problem. There was still weird behavior when editing existing applicatoins (some of the data was still not showing up in edit mode even though it was listed in display mode and I couldn't save my changes while editing because I received a message that such an application already exists).
But I was finally able to delete the existing applications without problems and to then create them again. Now everthing seems fine - onboarding works and the data connections to the backend are also working again.
Thanks a lot for your help!
Christian