Customizer does not work anymore

Hello, the Customizer does not work anymore as it did in the past. Here is a screenshot: https://prudentwater.com/wp-content/uploads/2021/01/Screenshot.png The website: prudentwater.com I have already deactivated the latest installed plugins but it did not help. It is quite strange how the Customizer behaves right now and I never had it before. Clicking on the tabs on the left side does not help anything. Might you have some solution? Regards, Ben