I just upgraded to SS 2.3.3 and installed the userforms module. An old form that I have shows up, but won't let me edit it. So I created a new dummy page, changed it to a userform page, then tried to add a field, but no luck. When I select the field type and click add, the browser gets redirected to /admin/EditForm and the page is blank.
Any ideas on why this would happen? Once I can add a form to a new page, I will deal with editing the old form.
Also note that the new userforms module is not compatible with the previous versions (for the most part) so you may run into issues updating existing forms.
Doing the /admin/?flush=1 helped clear up the blank page. I now see the green 'saved' text that shows up at the bottom, when clicking on the add button. But this still doesn't produce any form fields for me. I click the save button for the page, then refresh and still the form is blank.