I have same problem as you Heyme and I have tried same solutions as you. Still no success. Woocommerce/WordPress need to look at this issue and fix it asap. My shopp is temporary closed because of this.
A recap of the problem: “On sale” label shows upp, even though I don’t have any sale-price. Regular price is entered but not showing on page. Changes don’t apper after editing and saving product.. Everything goes back to what I had previous. VERY VERY frustrating. Please, anyone have a clue? Both WP and Woocommerce are up-to-date.