Cannot start Reporting Services 2005 under certain AD Accounts

We've still got a problem with this. I've just found this entry in ReportServerService_09_06_2013_11_07_44.log

ERROR: Exception caught while starting service. 
Error: Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerDisabledException: 
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. Check the documentation for more information. ---> System.Runtime.InteropServices.COMException (0x80090005): Bad Data. 
(Exception from HRESULT: 0x80090005)

Can anyone advise what causes this, and how to resolve it?