all my Products are on “Sale”

Sale price looks the same has the regular price. When you edit a product, is anything filled out in the sale price field or is it empty?

There’s a good chance the theme or another plugin is causing this. Follow this guide to find the likely culprit: https://docs.woocommerce.com/document/woocommerce-self-service-guide/#section-4

Thanks for your answer.

It was an other plug-in – just had to update this!