sysadmin user can't connect to explicitly specified database I noticed some Failure Audit errors, in the Windows Application event log of the server that my SQL Server runs on, stating:




Event ID: 18456  
Login failed for user 'DOMAIN\username'.  
Reason: Failed to open the explicitly specified database. [CLIENT: &lt;local machine&gt;]



The user mentioned in the errors was in the sysadmin server role, and I checked that the user had a default database associated with it. The errors recurred every five minutes. Does anyone have any insight into this issue?

The SQL Server service itself and SQL Server Agent run as that account.
I checked whether I had any SQL Agent jobs on the server that ran every five minutes, and found one that was failing.  When I checked the job steps, one of them had a blank Database drop down.  I selected a database from the drop down, and that corrected the issue.  Now, after finding the root cause, the error makes perfect sense: that user definitely couldn't open an undefined database.