I had the same issue and was able to solve it.
In my case, I had enabled the "Print As Image" option in the "Advanced Print Setup" dialog, which pops up when I press the "Advanced" button next to the printer selection dropdown box in the "Print" dialog. After enabling this option, the "Poster" button is greyed out. After disabling the "Print As image" option, the "Poster" button was available again.
