"The report server cannot decrypt the symmetric key used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content.
As the error shows, it is an access denied on a registry entry. A Process Monitor http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx trace can show which registry key needs permissions for the service account of the SSRS service.
Usually, it could be this –
HKLM\Software\Microsoft\Microsoft SQL Server\Reporting Services\Service Applications
A brute force change to permissions at HKLM\Software\Microsoft\Microsoft SQL Server by adding the desired user with Read (propagated to child folders) could show if that’s all that is required.
Assign the service account full permission or if it doesn’t work assign everyone full permission on the desired key.
There may be multiple keys which is giving access denied in the process monitor.
try to fix all of them one by one.