Error "page missing" when clicking "new" in a Web Form

I created and published a web form with the route book-seminar. Under /book-seminar I see the Web Form, essentially consisting of a button “New”. When I click the button, I get redirected to /book-seminar?new=1 which displays the “Page Missing or Moved” error message with return code 404.
What is wrong? Did I miss some setting? How can I debug this issue?

I didn’t find the error, but bench update did resolve it
(update to git HEAD 4e896b5b9ec61835273314be428099399a6a7936 on Nov 25 2017)