Recently we have encountered an issue where a domain admin (and local admin) account was trying to access the SSRS Report Manager and Report Server web sites and was getting the following error:
User DOMAINUserAccount does not have the required permissions. Verify that the permissions have been grated and Windows User Account Control (UAC) have been addressed.
The easiest solution to the problem is to disable UAC on that machine (a restart will be required).
After that you should be able to access to SSRS Report Manager and Report Server web sites.
If you need to keep UAC running, the following KB article details the configuration required for the installation to both install and run SSRS:
http://support.microsoft.com/kb/934164