edX "preview changes" button doesn't work

[usernamenumber]

In edX studio, if you create a unit and press "Preview Changes", it tries to go to a URL like:
http://courses/Some.org/xx101/2015_06/jump_to/i4x://Some.org/xx101/vertical/1519b7407dda462986cd29644e90873c

Note the lack of a hostname. Piotr says this might be because there's a separate hostname (virtual host?) that's normally used for previews, and it probably isn't getting set up for some reason.