I was looking at top URLs for 500s and it seems https://phab.wmfusercontent.org/ yields a 500 with
Unhandled Exception ("AphrontMalformedRequestException") This Phabricator install is configured as "https://phabricator.wikimedia.org", but you are using the domain name "phab.wmfusercontent.org" to access a page which is trying to set a cookie. Access Phabricator on the configured primary domain or a configured alternate domain. Phabricator will not set cookies on other domains for security reasons.
We could probably just redirect to phabricator's homepage from phab.wmfusercontent.org / @mmodell ?