Unable to access PBIRS with custom URL

Greetings.

I recently installed a PBIRS at a test site and configured a custom URL for the web service and web portal. Both the service and portal are accessible with the basic <localhost/ReportServer> and <servername/ReportServer> but when I try to access with the custom URL <xxxx.abcd.com/ReportServer>, I get stuck at the login prompt. The prompt just comes back again after I enter the credentials. Any assitance on why this is happening and how it can be solved will be appreciated.

TIA.