Ssologin redirect issue

See more:

SSO

At a customer location is someone tries to access our application directly we are supposed to redirect it to their SSO and that validates and returns it to our SSOLogin page that checks if the user is authenticated and then fetches the user name from the owin context and then loades the default page with appropriate previlages.

It was working fine but lately after a few changes it stopped redirecting back to our ssologin page instead it redirects back to out default page and give error 500.

The new web.config works fine with the old build, so it's not web.config or iis issue.

What's wrong?

Thanks

What I have tried:

We tried comparing all related changes and not able to find anything that might have caused it. We even tried to revert the startup.cs and SSOLogin.aspx.cs pages to the version that was working. Nothing works.