Review Board 2.5.6 shipped in a bit of a broken state, due to a packaging error that wasn't caught by our automated tests. This led to JavaScript failures on certain pages, like the New Review Request page.
Review Board 2.5.6.1 is out now and fixes the error. If 2.5.6 broke you, just install 2.5.6.1 and you'll be back up and running.
We're expanding our automated testing to ensure this particular problem won't happen again. Sorry for the trouble, and thanks for using Review Board!