Spinning icon shown forever when trying to add content to review via "Explore Repositories"

If you try to add content to a new review and select "Explore Repositories", a spinning icon is shown forever.

I'm using a brand-new test instance, with a very small repository (140 small commits) and 3 reviews created, so it's not a performance problem.
This is reproducible in Internet Explorer 9, Firefox 5 and Chrome 14, so it's not browser related as well.

It works if you choose any other option ("Browse Changesets", "Search for Files", "Pre-Commit" or "Attachments").

Please see the images attached showing the problem.

Nothing is thrown in the logs, unfortunately.