xp_logininfo needs permissins at the server level (if NT group) or the domain level (if domain group) to read the group. The SQL account probably has that. The other account...

Does the problem go away if you use SA or SQL Server Account as the owner? If so, the issue is not due to case sensitivity, it is because of...

Right click on SQL Server name in Management Studio and click properties.

Go to the Security Tab

Choose SQL Server and Windows Authentication Mode

Click OK