Hi Rikard,
thanks for following up.
Meanwhile I it could more or less solve that by myself.
In order to activate Google reCAPTCHA completely, the cookie for Google reCAPTCHA Service must be accepted. If the user opts out of Google . reCAPTCHA, it is not shown on the contact form. Instead it presents that “error” message.

One thing is open now: how/where can I configure the error message text? I would like to tell the visitor explicitely to accept the cookie.

Regards
Jörg