“The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content. ”
I’m busy configuring reporting for Dynamics CRM 2013 and came across this error with my SSRS. It turns out it’s because when I installed SQL 2012 I used the Local System account, and then switched it to a domain account so SSRS could work which then causes issues with the encryption keys. What you need to do is export and then reimport the keys.
Instead of repeating something already written, refer to this KB article for the fix: http://support.microsoft.com/kb/842421