I get following message as answer in firebug:
{"mailSent":true,"into":"#wpcf7-f566-p567-o1","captcha":null,"message":"Ihre Anfrage wurde erfolgreich gesendet. Vielen Dank! <br \/>Sie h\u00f6ren in K\u00fcrze von uns."}<br />
<b>Fatal error</b>: Call to a member function get() on a non-object in <b>/www/htdocs/.../wp-includes/cache.php</b> on line <b>113</b><br />