Some players are having issues logging into Pokemon Go thanks to a Google security issue. A recent update to Google's Safety Net is causing Pokemon Go to misidentify some devices as not being compatible to the game. Safety Net is a Google API that generally checks to see if specific Android devices are satisfying compatibility requirements. It's believed that Niantic has used Safety Net to see whether or not a phone is "rooted" (the Android equivalent to jailbreaking your phone), which could cause some stability issues and exploit security issues within the game. However, it appears that the new Safety Net update is even affecting phones that aren't rooted...at least according to their owners.
The good news is that the Safety Net issue appears to only be affecting a small number of devices. According to PokemonGoHub.net, the list includes multiple ZTE devices, the OnePlus One and OPO 2, the Redmi Note 3, the Redmi 4, the LG G3, and the Alcatel One Touch.
So what do you do if you have a phone affected by the issue? There's no current fix, but players on Reddit have said that restarting the app will fix the problem...at least temporarily.
We'll see if Pokemon Go's next update will fix this issue or if there's something else causing the issue on the users' end.