Have you checked http://status.nintex.com in that moment? Maybe the "Workflow Designer" service was facing issues? I'm looking at the action and it works just fine:
Do you face such issue with other "Body" fields? Maybe you can find something in the WebDeveloper console?
I think this issue is rather computer-specific - maybe it's because you have something blocking scripts, maybe browser related, don't know, but trust me - it works just fine