Hi Prasad, we had this EXACT issue after our SaaS environment was updated to 1811. After working with the support team we found it was the update that broke it as it forces SAML logons for admins, enrollments etc etc. The fix was to have a temp admin created, then logon and in directory service UNCHECK Admins under ' SAML' .
This should get you all fixed up and your admins logged back in.
- Jeff