Cannot submit form using reCAPTCHA

Hi!

I use the latest WordPress and Contact Form 7 version.

I use following contact form:

<p class="inputcf7">
<label>Vorname</label><span class="use-floating-validation-tip">[text* demo-firstname]</span>
</p>
<p class="inputcf7">
<label>Nachname</label><span class="use-floating-validation-tip">[text* demo-lastname]</span>
</p>
<p class="inputcf7">
<label>E-Mail-Adresse</label><span class="use-floating-validation-tip">[email* demo-email]</span>
</p>
<p class="inputcf7">
<label>Firma</label><span class="use-floating-validation-tip">[text* demo-company]</span>
</p>
<p class="cbcf7"><span class="use-floating-validation-tip">[checkbox* termsandconditions id:termsandconditions ""]</span> Hiermit erkl?re ich mich mit den <a href="termsandconditions" target="_blank">Nutzungsbedingungen</a> einverstanden.</p>
<p>
[recaptcha theme:dark]
</p>
<p><br></p>
<p class="submitcf7">[submit "Demozugang anfordern"]</p>

As soon as I place [recaptcha theme:dark] inside, the contact form cannot be submitted anymore. It shows the standard error “There was an error trying to send your message. Please try again later.”.

You can find the contact form over here:
https://iprot3.uidesign.at/

Any ideas?

BR
Stephan