Error in send form

I have a button “Test Tribune” on https://www.makkumfriesland.nl/agenda.php, which opens a form in a modal.
The form had two send mail server actions. One mail goes to me and one to the client. The mail to the client is in a template.
Now the mail to myself will send, but the mail to the client throws an error and will not send.
What have I done wrong here?

Appears to be in the template. When replacing it by “static” it will send.