502 Bad Gateway on redirect after successful login

Hello

I am using WP SAML Auth plugin with a custom developed Share point IDP. It was working fine for past 1 year it has stopped working now.
I am getting 502 Bad Gateway on wp-login.php page after successful login redirect. I want to debug what things are actually causing trouble.
Please tell me in which file or function i can find the code that runs after redirect from IDP login page. I have tried debugging do_saml_authentication function in class-wp-saml-auth.php file but i am not able to print response data from IDP on page.

Thanks
Sajid