during pentesting SOME site I have faced with file upload errors. It is 403 error Forbidden but the most interesting thing lies in the fact it is happening ONLY when I use burp proxy. When I am uploading passport/id document as a usual way(withou Burp Suite) - No error. 200 OK. When I am uploading with Burp Suite - 403 Error.
P.S.: Other site functionality/APIs work properly when I use Burp Suite
I have no idea why this 403 error occurs. Is this a bug or a kind of "protection"?
Then you don't have the authorization to set up a proxy in your OS. I guess you are using your corporate laptop. Try to change you network settings in your browser. If they are grey out then your organization has blocked that for you ( Bgroup rules).
I faced the same issue exactly and this was the problem. Hope that helps.