Well there are several plugins, ofcourse and I can list them here.
But why are other forms, like these, don’t behave like that? No errors, they just work like they should.
https://waidmannsheil.nl/contactgegevens/
https://waidmannsheil.nl/gratis-kennismakingsnummer-waidmannsheil/
The websites is https://waidmannsheil.nl/
The form that gives the errors after submitting is https://waidmannsheil.nl/prostaffteam/
Thank you for pointing me in the right direction.
I have solved this now.
In the menu I did not choose the page to add, but added a custom link.
This is causing a fresh load of the page.
