Contact Form 7 dont work

Hallo together,
I installed yesterday contact form 7 and WordPress 4.6 German.

The following formular dont work: http://www.antjes-ferienwohnung.de/_wordpress/kontakt/
It showes me after pressing the Send-Button the round two arrows all the time, it is not working.
I work with Strato Germany, the php-Version is 5.6.

Could you help me please.
Best regards
Stephan