I was trying out a few things, but eventually I brought all settings back to the previous situation.
However, when now a new User creates an Account, the app creates not 1 but 3 new records in the database of which two are empty.
Plus, the User is not added to a Role in the User Role database collection anymore.
Users can sign up with three different buttons. They will get a different user role depending on which button they click to sign up. Each button leads to a different screen and each of these screens has their own signup form.
I have now tested it with the Coach button and the result is the same as with the Founder button.
So, to my understanding, this is not an issue of the buttons or the signup form, but an issue with Adalo itself.