Error logging in with SSO web (Agent login)

Summary:

I am getting 'No file specified' after successful login from IDP

Content (required):

ACS url provided to IDP:

https://mysite--tst.custhelp.com/cgi-bin/mysite.cfg/php/sso/saml2/sp/post/acs.php

IDP redirects to this url https://mysite--tst.custhelp.com/cgi-bin/mysite.cfg/php/sso/saml2/sp/post/64c090c7354d7 which browser displays as not found.

Version (include the version you are using, if applicable):

22D

Code Snippet (add any code snippets that support your topic, if applicable):