a quick update: the game’s login servers are not operational at the moment. Logged in players can still play, but new players can’t log in.
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.
Niantic support’s tweet
We're currently investigating reports that Trainers are encountering login issues.
— Niantic Support (@NianticHelp) February 6, 2018