Workfront Fusion Github connection

Hi Team,
I've tried to create a connector with Github from Workfront Fusion. I've tried default and manual configuration(with client id and secret). I used Authorization callback URL as https://app.workfrontfusion.com/oauth/cb/github. As the result after confirming github permissions in popup window every time I receive error {"detail": "Unexpected token k in JSON at position 0","message": "Bad Request","code": "SC400","suberrors": []}. Could you please help with it?