@olaf: Well it depends on what changes you’ve made. I guess you’ve put it in style.css. What changes actually have you made? If you have’t made much changes in designs, I recommend you to get rid of all the styles from child theme styles.css. and just keep the functionality. It will fix your issue. Solving the issue after making child theme seems to be almost impossible cause we won’t know which change or customization is causing such issue. Hope you understand.
If possible, try creating a fresh child theme and customize it one by one checking frontend that it won’t break the responsive design.
Regards,
Mahesh