In certain circumstances users are unable to login to SAP using SSO when the User Principal Name (UPN) is in lowercase but are able to do so when the UPN is in uppercase (or vis-a-versa).
The release of Authentication Services 4.2 included Kerberos security enhancements which need to change to be case insensitive.
This issue impacts all versions 4.2 and up.
Change the case of the users UPN so that the user is able to login using SSO.
Downgrade to a 4.1.x version of Authentication Services.