You probably already tried this, but just in case... try clicking on 'Wallet' instead, it'll load to a different page with a 'Withdraw' link, which I have found to always work even when the one on the summary page doesn't (sorry, slipped my mind earlier otherwise I would have mentioned it before).

Also, if that doesn't work, try switching to the 'Classic Site' (there's a link on the bottom left-hand corner after you've logged in).