I found the bug. The problem is that the form uses the email address of the person filling out the form. The send from address is therefore the e-mail address from the form instead of the e-mail address of the website.
Because the user's e-mail address is not authorized to send in Sendgrid, nothing is sent. The moment I authorize the address in Sendgrid, the sending works. You have probably also validated your e-mail address in Sendgrid in the past, so that’s why it works for you.
The send from address must at all times be the e-mail address registered in the website. This can never be another address because it will then be seen as SPAM because of Dkim, SPF and DMarc.
In addition, the subject of the mail is also used as the sender's name, which is incorrect.
Can you provide me with a fix for this bug?