I too just got this same error when attempting to change the description of a label. My workaround was to delete the label and recreate a new one.
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?
This just happened to me trying to resize much of my label text during a form makeover session. Recreating the problematic label is the best solution!
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.