Only one user cannot log into an app via Google ID Authentication
Worked this out. Whoever set up the user's ID originally had a capital letter in the ID - but not in the Email addr so this wasn't showing up anywhere.
At work we run a python application where users log in via their google account. 
One user gets an "Error logging in" message on any instance, this doesn't replicate on any other instance. 
The app was made by a third party and they can't tell us why this happens. Is there a debugging tool or something that comes with Google auth that could be used to trace where the failure is happening?
Thanks in advance. If any more technical details are needed please let me know. I'm not very familiar with how all this works.