I'm not sure what I broke, but suddenly my registration form is no longer displaying. I wonder if it has anything to do with my inadvertently selecting a prerequisite event.
How do you undo selecting a perequisite event anyway?
Yes, you can see in the url that a prerequisite selection has been made. The page is "blank" because you have no prerequisite message created in your configuration.
To deselect the prereq item, go into your event and use CTRL-Click to deselect.
This one just bit me too. It also throws the error message you put in for having to be signed in to register even though it is a public event. I fixed the selection of a prerequisite event (dummy me, I picked the event itself so you had to have attended the event you are registering for to register... duh) and presto, the public event registration worked.