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The servers may be down for some, which you can see here also by the comments on the page:
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Login and Server issues seem to be logged at almost 50/50, so it's a combination of both.
This is if you cannot login to Pokemon GO, or have any BOT issues, or you can load the game but there are no pokemon or pokestops.
Edit: It appears to have been fixed, Necro, Haxton and Nox all work now.
Edit 2: Now they are down again?
Just be patient with the servers.
Edit 3: As of right now, Logging into NOX works fine, Haxton works and Necro works too, no need for a VPN.