Hello Stefan,
Thanks for writing to us.
It might be the issue of custom CSS code, the slider height issue or section padding issue.
Please go to your Page -->Edit page--->Section
and Inspect

Go to the Padding Settings and set padding-top
and padding-bottom
as 0
Screenshot
If you are not able to see padding settings, click on the Setting icon and enable the “Advanced Mode” as “ON”
Screenshot
If it doesn’t help you, please share your details in a secure note. Please provide the following details
- WordPress Login URL
- Admin level username and password
You can find the Secure Note button at the bottom of your posts
Thanks