I don't know the reason that this happens, but I can tell you that if you click Done Editing at the top of your article, then click Edit again, when you go into the Poll module, you will be able to type in it.
I have had this happen with Photo modules, Poll modules, and callout modules. Usually this happens after I have been working on a hub for a while, but it has happened when I go into the module after making a change to a different module. All I can tell you is that getting out of editing mode, then back into it makes it possible to do again.
I hope this helps.