opstore css error

WEB: http://www.forschnit.com
Theme: Opstore Lite

In WP admin when I am opening the appearance – customize options the site is not loading properly, it seems a CSS issue and I also attached a screenshot with the Chrome dev tools showing also missing CSS issues. When editing a product and adding a new media the editor page is also not loading properly, it has a very similar css issue and I also attached the Chrome dev tools output showing CSS issues.
When I install the WP Debugging plugin the site fucntions properly.

Please let me know if you need any additional info to resolve the issue. Many thanks, Tamas