Offline issue update
After further investigation, it appears that signing in to Reviewable in multiple contexts (browsers, profiles, etc.) causes all but the last sign-in to go offline. A workaround is to sign out quickly before the offline modal appears and sign in again; this should fix the current session while breaking any others.
I'm working on a better fix, of course. Also, if you're seeing the issue without having multiple concurrent sign-ins, please get in touch so that I can get a better idea of the extent of the problem. Thanks!