Single Sign On Test connection is successful but during actual login user is immediately redirected to logout
Checking the system logs if you see the below error:
Caused by error in Installation Exit: 'MultiSSO' at line 1
==> 1: gs.include("PrototypeServer");
This indicates that SAML response does not contain "sessionIndex"
Sample attribute in SAML response that contains the SessionIndex
<saml2:AuthnStatement AuthnInstant="2019-11-15T20:05:07.158Z" SessionIndex="SNCf58ab2979adc3a4d30fa487bafb0847b">
Check with the IDP admin and make sure that sessionindex is returned in the SAML response.