Recaptcha refuses constantly to work

Using a simple basic domain and setup WP with standard features to try the recaptcha Plugin delieveres consequently the same error, even if the key check inside the plugin says “All Okay”.
There is NO Subdomain or any other structure which can make it difficult, its only a simple DE domain which tells me fehlerhinweis für inhaber der website invalid domain for site key.
But it is not its clearly defines at google recaptcha, not used for another domain NO only for one domain.
So I think there is a clear improvement error which has to be found.
In the meantime we will uninstall the plugin otherwise the website and so the business is interupted.

Let me know when the error is fixed.
Thanks