Hi,
Thank you, I have found the problems, it is only the configuration.
1. Issue: The abfahrtsart filter. Items matching that filter does not have either of the dates set, so setting any of the date filters will automatically give you no results whatsoever. You will have to either remove this, or set a value for each element.
2. Issue: The abfahrtsdatum filter Date storage format. It should be the default ACF Datetime field, as it is a datetime field as configured in ACF.
The other filter storage format is okay, as it is ACF Date not datetime.
Changing those will resolve the issue 🙂