I suspect the issue here is not the DiscourseConnect security fix but rather the nginx change. On tests-passed we had to make a followup on Thursday because it was causing problems on some environments and another user on Github noted CSRF issues.
I have a backport ready for that: FIX: Simplify nginx config change (#30383) by pmusaraj · Pull Request #30410 · discourse/discourse · GitHub, it should be merged shortly (once someone in the team approves it, though it’s Sunday for most people). You’re welcome to give that branch a try with your SSO set up @mentalstring