DPM 2010 SQL Reporting Error (rsProcessingAborted/rsErrorOpeningConnection)

An error occurred during client rendering.
An error has occurred during report processing. (rsProcessingAborted)
Cannot create a connection to data source ‘DLS’. (rsErrorOpeningConnection)
Cannot open database “DPMDB” requested by the login. The login failed. Login failed for user ‘<DPM Server Name>\DPMR$DPM’.

SQL Server Reporting Services (MSDPM2010)
Event ID 0

Failed to unload app domain DefaultDomain. The following exception occurred: Error while unloading appdomain. (Exception from HRESULT: 0x80131015).

• Open Microsoft SQL Server Management Studio. Expand Security, Logins. Right click <DPM Server Name>\DPMDBReaders$DPM go to Properties.

  

• Click User Mapping and select DPMDB from the right pane. Assign db_datareader database role membership.

  
  

• Now try to generate a report.