Small Update from my side. Was testing the last hour and was able to reactivate the repatcha now in the contact form. However i dont understand why it is only shown correct in the contact form when i am logged in as member on our pages. If i log out, it shows the error for site owner: Invalid site key
Then i tried again to acivate the WP Membership Recaptcha Plugin and enabled in for all forms which are available in the Plugin setting. However its is not shown in the registration form for example.