When performing the callback, Google gets the following error:
401 – Unauthorized: Access is denied due to invalid credentials.
You do not have permission to view this directory or page using the credentials that you supplied.
I have not found the direct solution, but the workaround I have come up with is to put the page it calls back to in its own directory and disable basic authentication on that folder. The call back then completes. I recommend using a randomly generated folder name to prevent it from being stumbled upon by nefarious individuals.