This has occurred for the last few days; as if a server-side update is taking place that updates one server at a time but forgets to transfer users off of it first.
Side effects include the aforementioned 501 error (inability to login), alliance screens showing zero members, affected users showing up as not logged in for ~50 years (18008d 20h) at level 1 with 0 medals, and crashed war attacks.
On the bright side, it appears to only happen once per user and for about 30 minutes... but time will tell.