Update: Problem Resolved
Thanks again to everyone who offered input – and special thanks to @pfaffman for immediately pointing us in the right direction.
We’ve now identified and fixed the issue:
The setting allow new registrations was indeed disabled in our hosted instance.
This blocked OAuth-based signups even though Patreon authentication itself was working properly. Here’s what we’ve confirmed:
We successfully tested the entire signup and login flow using a completely new Patreon account.
The user who originally brought the issue to our attention was also able to log in without any problems afterward.
Everything is functioning correctly now – users are being created and are added to the appropriate Patreon group as expected.
Thanks again for your help and support!