Have exactly the same problem... every 15 minutes 24/7.

Filling the log with this:

Error: 18456, Severity: 14, State: 38.

Login failed for user 'NT AUTHORITY\SYSTEM'. Reason: Failed to open the explicitly specified database 'DATABASE_NAME'. [CLIENT: xxx.xx.xx.xx]

for every database in the server... Which is drowning the log, making it hard to find the useful messages...

Looks like it started after patching the server...

This is the current version:

Microsoft SQL Server 2012 - 11.0.2383.0 (X64)

Oct 5 2012 19:35:54

Copyright (c) Microsoft Corporation

Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1)

What internal function in SQL Server has this behavior?

/Par