I have always integrated a SAML IDP with Salesforce and I'm trying OIDC now.
I have created a Salesforce OIDC app in my IDP. I took the clientID and clientSecret, and then created a Auth. Provider in Salesforce. I copied the Callback URL and SSO URL back to my IDP.
But my tests both with Test-Only Initialization URL and SSO from my IDP are not working.
When I open the Test-Only Initialization URL in browser, I'm redirected to my IDP. After logging in, I'm redirected back to Salesforce and this page is displayed.
For SSO, there seems to be a problem with my registration handler.