The only thing I can think of is that your form is saved as a Static
form instead of Dynamic.I tried saving your form as Dynamic, but it
won't let me as you are using a PDF as background art. I think you'll
need to start a new form and make the whole thing in LiveCycle.I copied
and pasted all the fields from your form to a new blank form, saved it
as a Dynamic PDF, Reader Extended it with Acrobat and get no error. I've
attached the file I did.