Login failed for user 'NT Authority\SYSTEM'. Reason: Failed to open the explicitly specified database.

We had an issue where this message was appearing in the SQL Server error log every 5 minutes.

It can occur when SCOM (Microsoft Systems Center Operations Manager) attempts to poll SQL Server database information, but is not correctly configured.

To resolve this, create an SCOM-specific SQL server account with permissions on each database, then configure SCOM to use that account.