Hi @Helga
The bug was reported as resolved after the June 2023 update to ArcGIS Online, and I haven't seen the behaviour in my testing since.
I just tested your service and it doesn't seem like the behaviour falls under that BUG-000156736. If I add the Fauna_Flora_Habitat_Gebiete layer as a WFS item in my content, then open Map Viewer I am able to add that layer via the Add layer option
It does look like there are projection issues happening though.
Where are you running into problems?