Theme not responsive any more

Two of our websites are running under Grecko 4.4 and WordPress 5.4.2 - since about a week ago both of them are not responsive any more. The URLs: www.australia-news.de and www.diewoche.com.au - Now almost unusable on a smart-phone.
We most probably made a mistake when working in the back-end. In “Theme Options” / “Customization” / “General” / “Responsive Layouts” it is set to “yes”.
What else could it be?
Thanks
Regards from Sydney
Dieter