Reporting Service 2008 Microsoft.ReportingServices.WmiProvider.WMIProviderException

I was changing the Service Account to use Build-In Account to Local System.  In "Backup Encryption Key" prompt, I entered file location and the password.  After I click on OK to the "Backup Encryption Key" prompt, the following error message shows up:

Microsoft.ReportingServices.WmiProvider.WMIProviderException: 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 want to ask what is this mean?  And how to resolve it?

I am using Microsoft SQL Server 2008 Reporting Service.

Thanks

Peggy