When the user can't login, either from Windows Client, Mac client or Web portal, the first thing to check is always go to the web portal and see if the user can login to the web portal.
1. If the user can login to the web portal, now focus on the client, whether it be the Windows, Mac or others.
2. If the user can't login to the web portal, check the error message. Either Login_Failed, Auth_Failed or Not_Found.
<To_Be_Continue : with full diagnostic script for login failed issue>