Hi Jan,
Thank you for writing in, besides the child theme style.css file, did you add custom CSS somewhere? specifically on the Elements CSS area? Can you recall all your custom CSS and check for syntax error here.
If that does not work, try switching to parent X theme there might be something on your child theme that is messing up.
If the issue still persists, please do a testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Let us know how it goes,
Cheers!