Upgrade to Gitlab v10 breaks custom omniauth integration

Up until v9.5.9 we had a working omniauth provider to allow users to login to GItlab via our own oauth2 server. After upgrading to Gitlab v10.3.3 the integration no longer works.

The error we get is:

Could not authenticate you from OurAuth because "Invalid client: client authentication failed. {"error":"invalid client","error description":"client authentication failed."}".

This is the only method to login to the server. I’m not sure where to begin in trying to resolve this issue. But I suspect that our custom omniauth strategy is no longer compatible.

Is there anyone that might have some knowledge of using omniauth and custom strategies with Gitlab?

After some more digging the answer was finally discovered. The omniauth strategies were indeed the culprit. If anyone else has this issue, be sure that your strategy includes:

  def callback_url  
    full_host + script_name + callback_path