Trying to authenticate via OAuth2, getting "/ does not start with ." error

[Dmitry Malinovsky]

Hi, I follow a 3-step OAuth2 flow. When the user authenticates on bitbucket and grants access for the application, it's being redirected to the site which tries to authenticate the user on backend given the provided code.

This is when I receive the following 400 error:

 {'error': 'invalid_request', 'error_description': '/ does not start with .'}

I have no idea how to interpret that message. What's exactly wrong? What exactly must start with a dot?

Thanks in advance.