Worked well until today, I changed nothing and currently an empty form is displayed :(
This happens when you select a person in the person field. This person field will disappear and will not allow you to change you selection. Some users will elect to go back one page on the browser and re-enter the form. When they do that, it will show a blank form.
I'm sorry you're running into this problem! It appears we're having trouble recreating the same issue on our end. Could you please send us a quick message to email@example.com with your form URL so that we may investigate further?