We've identified a bug that prevents the new editor from loading, instead displaying a spinning circle. The underlying cause of the problem has been identified, and we are deploying a fix now. As a temporary workaround, you can still post using the old post editor (the preference can be changed under the Settings | Basic tab).

Thanks for your patience, and things should be back to normal shortly.

Update 1/6: The underlying issue has now been resolved and the new editor should be working again.