I had to replicate a bunch of fields in a Zoho Creator form (i.e. I'd created them for client 1 and then needed to recreate them for client 2). I figured it would be faster to go into the form definition and copy and paste elements there rather than creating them via the UI. It ended up taking longer haha. For some reason the form definition seems to be very temperamental. I can't precisely tell you what ended up allowing it to work, but here are some things that you definitely can't do:
1. Have a field with the same ID as another field (display name can be duplicate though)
2. Have any comments in the form definition.
I was plagued with "Error: null" (gotta love ZC's highly descriptive error messages!)