Error trying to view a different user's profile

I can view my own profile just fine, but if I try to view anyone else's, whether they're my friend or not, there's an HTTP 503 response and the following message shows up:

"Temporarily Unavailable

The website that you're trying to reach is having technical difficulties and is currently unavailable.

We are aware of the issue and are working hard to fix it. Thank you for your patience."

I've seen this for a few days. It only seems to be a problem if I'm logged in; if I'm not logged in, I just see a stub profile with a note about needing to log in to see details.