Since I have not received the certificates with the contact formular people I found out the right contact for certificate is this email address:
certification (at) qt.io
From there I got the information that there is currently a bug in the Persona VUE System. When you finish an exam it states you get the certificate in 4 to 6 weeks but actually you never receive it because:
Persona VUE does not notify automatically to Digia
According to Digia they were thinking no one took the exam currently because of COVID so they were not aware that the notification system between Persona / Digia was broken.
I had to send my passed exam dates so they manually start printing the certificates.
Some days ago I received my certificates but printed with the name wrong. If you have special letters in you're name like in my case the german "ß" be aware that they maybe get printed wrong.
So now I wait again for the certificates...
But I have to say response from certification (at) qt.io is fast. I wish they would have stated this contact address somewhere more obvious....