This is the solution:

1. (refer to the Jin Chen's reply in the post):

https://social.msdn.microsoft.com/Forums/sqlserver/en-US/7f477018-aa14-4db0-8989-ce763b122a2a/sql-server-express-2008-reporting-services-rsaccessdenied-error?forum=sqlreportingservices

2.(followed by) Configure a Native Mode Report Server for Local Administration (SSRS):

https://msdn.microsoft.com/en-us/library/bb630430.aspx

3. (related)

http://stackoverflow.com/questions/5651245/can-not-access-report-manager-url-in-reporting-services-configuration-manager

I followed the steps from 1 and 2 and it works all fine.