SSRS Scale-Out Deployment: cannot decrypt the symmetric key
I work with Roger and the issue turned out to be after a little work that the second server was a cloned image that had been OBEd so it was a "different" server from the server SID and identity. However, the install of SQL 2008R2 SSRS was already on the server. The SSRS install creates a unique InstallationID (GUID) and this is used to identify the SSRS instance in a scale out of SSRS. So both servers have the same InstallationID and that means they both appear to be the same server and thus the key is the same for both and it cannot backup or restore the key since it conflicts with an existing key in dbo.keys in ReportServer DB. The InstallationID can be found in rsreportserver.config file, it will also be in the registry in many, many locations and trying to find and fix them is not a feasible choice.

The solution is to remove and then re-install the SSRS instance on the second server so that it will have a new InstallationID and crypto key. Then it can be added to scale out installation.