We just had this same error reported by one of our form developers, She was just trying to edit a label in the Nintex form as well. We are on SP2013, NF2013 latest release as well. Did anyone ever provide a solution to this problem?
Not sure why the error occurs. However if you disconnect the label from the control so it's not associated (Associated control = Not connected), you'll then be able to edit and update your label without recreating it.