Thanks for getting back at me Shameem.
I tried the following:
1. Theme switched to Storefront.
With this theme I have no option to edit my products. The form in the woocommerce products background is not shown.
2. So I switched again to the Woodmart-theme.
3. I deactivated each plugin separately, got back to the product and tried to edit the variation. Unfortunately, nothing changed.
Can I do something else to get this bug eliminated, any suggestions?
Does anyone has some ideas how to solve this? Is my thread in the wrong forum for that?
Appreciate any help, thank you very much.