[otrs] Open email attachment on ticket redirect to a login page

Hi I just upgraded from version 2.3.4 to 2.4.7. Most things work for me except , when I try to open an attachment on a ticket, it will be redirected to a login page. I tried login , select a role then the attachment can be downloaded. The previous version 2.3.4, didn't have this problem . As clicking on the attachment will just open the document. My system is running on CentOS 5.4 , apache and MySQL Can anyone advice on this very bizarre problem. Please help. Thanks xiaob