We started having same issue since last Friday (Dec. 15, 2016) after working fine for months. Same user can login via oauth on one wifi, but not on the other, or even on mobile cellular network. And even when it worked on that wifi at one point, it does not work at other times. We have not found a workaround to fix this, reported to Instagram 24 hours ago, waiting...I tried to login to my account on instagram.com first, then go back to our site, browser would log in automatically. So far, we don't have a solution yet.