We at smartmetrics.co experience the same issue. We noticed that issue affected not everybody, many customers still can use oauth.
We contacted Instagram and Facebook support but got no answer.
Apparently it happens from time to time, there are some posts about this problem on StackOverflow. No answers though.
We tried many things, including resetting our secret. It's working now, but it's hard to tell whether our actions had any effect.
It's a bit frustrating with no reaction from Instagram/Facebook and not even an entry on the status page.
Instagram had a long period of time it went down yesterday I'd be surprised if they aren't linked.
At least not in all cases, i.e. we tried production, staging and an instance running on localhost. Private mode usually changed in which places the login worked, but it never helped for all three.