At a customer of ours we have installed a SQL 2012 R2 server for all System Center Products including DPM 2012.
After installing the SCCM 2012 Reporting functionality on the SQL server the eventviewer flooded with:
Log Name: Application
Event ID: 18456
Task Category: Logon
Login failed for user ‘NT AUTHORITY\SYSTEM’. Reason: Failed to open the explicitly specified database. [CLIENT: 126.96.36.199.1]
After some investigation we found that the SCCM 2012 reporting installation messed around with the security rights of the System Center SQL database.
The NT Authority\System account was removed from the SYSadmin role
Add the NT\Authority\System account to the Sysadmin role