Django Social Auth Debug HTTP Error 400: Bad Request (linkedin)
Quite old question, but I got the chance to debug this issue today and the error was cause by server datetime mismatch with current UTC value, updating the server date fixed the LinkedIn issue.
I'd debug this capturing the network traffic and looking what is wrong in the request. You can use for example Wireshark to capture and browse it. This will at least give you idea which part of OAuth process is failing.