Hello Christian,
Sorry for the inconvenience you're experiencing. It appears the issue was caused by a translation error in your form builder header on the Kontakt page. The incorrect configuration was:
Antwort an: {{email}}
Absendername: {{first-name}} {{last-name}}
Cc: {{email}}
I've corrected it to the following:
Reply-To: {{email}}
Reply-name: {{first-name}} {{last-name}}
Cc: {{email}}
I have fixed this on your Kontakt page. Please check now to see if the problem is resolved. If the issue still persists, please let me know and I will investigate further.
Best regards