Is data from Stellplatz being lost? I don't see a refresh action on the microflow so any data that does get changed won't show up on the page until you reopen the page with that specific object.
And is the error you get a generic Mendix error? Does the stacktrace show anything useful? If you could copy the first 10 or so lines, that would help troubleshooting.
Additionally, the change action in your first microflow has commit set to yes so you don't need to commit the object again with a commit action.
The only reason why objects would be deleted is because you programmed it so. Double check delete behaviours. Could it be that some object is deleted which due to the delete behaviour deletes other objects? Otherwise show us some screenshots of your page and domain model. Because with the above text it is hard to pinpoint you in the right direction.
This is my Domain model
This is my first microflow
The second MF
I agree with Jim--
Benjamin, can you please share the error message (and stacktrace) shown in the Studio Pro console when you see the error message? That will help us understand what might be happening.