Invalid response encountered when trying to log in with OIDC

[axanite]

Have tried to follow the code and the .env example to set up OIDC. I managed to get it going to my OIDC provider (using OAUTH via Invision Community) and it authenticates as me but the callback comes back saying 'invalid response encountered'. I've tried to look at the logs with debug mode turned on and it doesn't say anything about the error - what should I be looking at?