The Pokemon Company and Niantic have confirmed that the issues causing the server problems in Pokemon GO have been identified. Now that they know what’s wrong, the development team is working on getting things sorted as soon as possible. If you’re still getting the error message, “Unable to authenticate. Please try again,” you can rest assured that a fix for this is also in the works. Pokemon GO was recently released in more than 25 countries as a free download for iOS and Android devices.
Pokemon GO server problems have been identified
Pokémon Blog 
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