Why does Instagram periodically refuse to sign in via OAuth?

Question

OAuth

I have my app running through the Instagram API, which has been peer reviewed and granted permissions to access "public content". For some time now, problems have begun to work in the application: when authenticating (server-side), I receive a code that can later be used to obtain an access token. Further, as per the instructions, I make a request to /oauth/access_token and pass in the parameters:
client_id=MyClientId
client_secret=MyClientSecret
grant_type=authorization_code
redirect_uri=MyRedirectURL
code=ObtainedPreviouslyCode
In response to this request, I should receive an access token from Instagram, but in some cases I get 400 Bad Request. Moreover, I can’t find the cause of the error, I can receive it in the morning, and in the afternoon it’s normal to log in (the request will be exactly the same). Some of our clients (users of our application) work normally, and at the same time, the other part cannot be authenticated in any way. Sometimes you can log in if, for example, you change your Internet provider or user (or better, both at once). The bad thing is that this problem is not predictable, and the commercial use of the application is a big question.
Tried a bunch of different advice from stackoverflow, nothing helps. Instagram support is silent, they sent several requests.
The most plausible theory at the moment is that Instagram for some reason bans users by the combination of Username + IPAddress + ApplicationID. But neither the reason for the ban, nor the very fact of its installation can be found out.
What are the options for such an error? Or can someone tell me how to reach out to Instagram support?

Answer 1

I figured out that Content-Type needed to be changed in the POST request.