Can I claim the loss was in 2013, because I attempted to withdraw my funds back in 2013 (late 2013), and they refused to send them because they were already insolvent by then?
Unfortunately the answer is no, as the date of the official bankruptcy would be used. But we also don't know how if/how they intend to restructure or repay. So even for 2014 it's unclear as you don't know how much you are getting back. If none or less, you'll likely claim it as a casualty loss in 2014.