This is rather a cryptic error message that you might get when installing the SCOM reporting component even if the SSRS is configured fine and the encryption keys have been regenerated:
Message:SRSPolicySetter SoapException, now throwing:System.Web.Services.Protocols.SoapException: 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. ---> Microsoft.ReportingServices.Library.ReportServerDisabledException: 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. ---> System.Runtime.InteropServices.COMException: Keyset does not exist (Exception from HRESULT: 0x80090016)
at Microsoft.ReportingServices.Library.ReportingService2005Impl.ListRoles(SecurityScopeEnum SecurityScope, String Path, Role& Roles)
at Microsoft.ReportingServices.WebServer.ReportingService2005.ListRoles(SecurityScopeEnum SecurityScope, Role& Roles)
[19:31:16]: Error: :StackTrace: at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object parameters)
at Microsoft.Reporting.Setup.ReportService.ReportingService2005.ListRoles(SecurityScopeEnum SecurityScope)
at Microsoft.EnterpriseManagement.OperationsManager.Setup.ReportingConfigurationHelper.ReportingConfiguration.SRSPolicySetter(String fullSRSURL)
-check if the datawarehouse accounts (read/write) are part of the local admin group on the reporting services server and DWH SQL server.
-try to use an account for the SCOM reporting component installation that is SA on the DWH DB and Report DBs, SCOM admin and local admin on the SQL server. Enable this also in SSRS as an execution account.
-remove any SCOM reporting component that might be a leftover from a former installation.
-recreate DWH read/write accounts.