I'd like a solution to this as well. We're running a live event and need to add fields ! We can't edit an old field to serve as a new field either. It looks okay when attempting to edit but it won't save.
We have also noticed that the placeholder [ALL_FIELDS] is showing up blank in the Custom emails going to registrant and to Administrator.
Please don't make us switch to eventbrite (shudder). We've depended on DTRegister for nearly a decade.