Portal Logon page in EP6

I am on EP6 SP2 and currently having issue with the portal logon page. When I try to access the portal via the full register domain name, the portal page seems to go nowhere - similar to a refresh after I clicked 'Log on'. For example: http://www.mycompany/irj/portal

However, once I am in the server, I was able to log into the portal successfully via http://localhost/irj/portal.

Can anyone tell me where I should look to correct this problem? Any help would be appreciated. Thanks.