a quick update: the game’s login servers are not operational for Android players at the moment. Some logged in players can still play, but new players can’t log in and there are reports of people getting kicked out of the game.
Niantic has already acknowledged the issue and announced they’re working on a fix for the problem. Early reports indicate that it’s a Saftey Net issue (Google app safety network infrastructure).
We’re updating this article as new info comes in. If you lost an Premium Raid pass, remember you are eligible for a reimbursement.
UPDATE: Resolved now.
Niantic support’s tweet
We're currently investigating reports that Trainers are encountering login issues.
— Niantic Support (@NianticHelp) February 6, 2018