SCCM 2012 R2 - Reporting Services Point - Failed Install "The DataSource does not exist"

Symptoms:
You have SSRS installed on a different server from your SCCM 2012 R2 install. You can access the default SSRS site in Internet Explorer using the default url (http://reportservername/Reports). In SCCM, you add a new Site System and select the Reporting Services Point for installation.

On your SQL Reporting Services server, you encounter a whole bunch of error message in the srsrp.log:

Set configuration
Check state
Check server health.
Successfully created srsserver
Reporting Services URL from Registry [http://sqlreportserver/ReportServer_SQL2012/ReportService2005.asmx]
Reporting Services is running
The DataSource does not exist.  
[sqlreportserverlocal\SQL1] [SCCM_XXX] [ConfigMgr_P01] [sqlreportserver.LOCAL]
[SQL2012] [1] [] [XX\SCCM2012Server]
[1] [0]
Confirmed version [11.0.5058.0] for the Sql Srs Instance.
System.Web.Services.Protocols.SoapException: The item '/ConfigMgr_XXX' cannot be found. ---> Microsoft.ReportingServices.Diagnostics.Utilities.ItemNotFoundException: The item '/ConfigMgr_XXX' cannot be found.~   at Microsoft.ReportingServices.Library.ReportingService2005Impl.GetProperties(String Item, Property[] Properties, ItemNamespaceEnum itemNamespace, Property[]& Values)~   at Microsoft.ReportingServices.WebServer.ReportingService2005.GetProperties(String Item, Property[] Properties, Property[]& Values)
Extract resource language packs if exists
Loading localization resources from directory [D:\SMS_SRSRP\SrsResources.dll]
Looking for 'English (United Kingdom)' resources  
Looking for 'English' resources
Found resources for 'English'
System.Web.Services.Protocols.SoapException: The item '/ConfigMgr_XXX' cannot be found. ---> Microsoft.ReportingServices.Diagnostics.Utilities.ItemNotFoundException: The item '/ConfigMgr_P01' cannot be found.~   at Microsoft.ReportingServices.Library.ReportingService2005Impl.SetProperties(String Item, Property[] Properties, Guid batchId)~   at Microsoft.ReportingServices.WebServer.ReportingService2005.SetProperties(String Item, Property[] Properties)
System.Web.Services.Protocols.SoapException: The permissions granted to user 'NT AUTHORITY\SYSTEM' are insufficient for performing this operation. ---> Microsoft.ReportingServices.Diagnostics.Utilities.AccessDeniedException: The permissions granted to user 'NT AUTHORITY\SYSTEM' are insufficient for performing this operation.~   at Microsoft.ReportingServices.Library.ReportingService2005Impl.CreateFolder(String Folder, String Parent, Property[] Properties, Guid batchId)~   at Microsoft.ReportingServices.WebServer.ReportingService2005.CreateFolder(String Folder, String Parent, Property[] Properties)  
(!) SRS root folder was reported missing
Failures reported during periodic health check by the SRS Server sqlreportserver.LOCAL.
Registry change
Waiting for changes for 1 minutes


Elsewhere in SCCM, the Site Status > Reporting Services Point says:

SMS_SITE_SYSTEM_STATUS_SUMMERIZER: Site System Status Summarizer detected that the availability of the "Reporting services point" role on server "\\NA-SQLR01.in.tna.local" has changed to Failed.

And the last message from the SMS_SRS_REPORTING_POINT Component Status is "Site Component Manager successfully installed this component on this site system"

Also, when you open the SSRS website you notice that no SCCM folder has been created:

Cause:
Insufficient permissions on the install account. Assuming the Reporting Services Point is installed on the same server as the site server role (which it will almost always have to be), then the local System account will need the appropriate security rights over SSRS since this is what install and configures the Reporting Services Point.

Resolution:
Uninstall the Reporting Services Point using SCCM admin console.

Open up Internet Explorer and navigate to the default reports page. From here you'll need to make an additon to the following areas:

Add the NT AUTHORITY\SYSTEM account to the Permissions areas in:

Site Settings > as a System Administrator > Click Apply

Folder Settings >  as a Content Manager > Click Apply

Re-install the Reporting Services Point and then keep an eye on the srsrp.log, the errors which previously popped up should now be conspicuous by their absence:

Furthermore, the ConfigMgr_XXX folder should now appear in the SSRS website:

After a couple of minutes, if you switch back to your SCCM admin console and navigate to Monitoring > Reporting > Reports you should see a whole bunch of reports have appeared: