Hi hendrikdobbert,

1. Please edit each Slide, in the Slide Settings you can change background image for mobile devices https://prnt.sc/YCjOGAcmfdrk

2. Navigate to Slides > Sliders, click edit button on the Slider and you can change the height of slider on mobile devices as well https://prnt.sc/WEJaOQATKoOI

If it still doesn’t work, please provide temporary wp-admin access to Private Content area, I will take a look.

Regards,