sijad Two things in triple-checking this:
1.) I had a space in the beginning of my client secret in the extension settings, which I thought I had resolved - but apparently it hadn't saved the change the first time i saved it.
That seemed to stop the error I posted above.
2.) I go between Firefox and Chrome, and was using both to test with similar results - Now if I attempt to login it processes my GitHub authentication, but indeed it returns me to the sign up modal with my GitHub username in place, and it insists I enter an email. Entering an email to continue, it returns an error and does not proceed.
I think you have hit the nail on the head though, as I do have Flarum installed in a sub-directory.
