I just realized my SSO is broken, but I don’t know when it broke (possibly when updating from beta3 to beta4?). Nothing has changed AFAIK on the SSO side.
When I click on Log In it redirects to the SSO site correctly, sign-in works there, but the final redirect to Discourse fails: I get a blank page that just says “Error updating information, contact site admin” (the response headers indicate a 500 error). The logs in the admin interface are completely empty.
How do I debug and fix this?