SQL 2012 error 15404

I have a DBA who is getting this error from sql 2012
 SQLServer Error: 15404, Could not obtain information about Windows NT group/user 'DOMAIN\USER', error code 0x6e. [SQLSTATE 42000] (ConnIsLoginSysAdmin


This is the error from the applications log on Windows 2008 R2 SP1

Log Name:      Application
Source:        SQLSERVERAGENT
Date:          5/8/2013 1:00:00 AM
Event ID:      208
Task Category: Job Engine
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      "SERVERNAME"
Description:
SQL Server Scheduled Job 'Backup.Transaction Log Backup' (0x73F40EB3D55EFB47A865552850B3A8B7) - Status: Failed - Invoked on: 2013-05-08 01:00:00 - Message: The job failed.  Unable to determine if the owner (DOMAIN\USER) of job Backup.Transaction Log Backup has server access (reason: Could not obtain information about Windows NT group/user 'DOMAIN\USER', error code 0x6e. [SQLSTATE 42000] (Error 15404)).

After further investigation He had updated SLQ 2012 to SP1 on May 7th and added the Hotfix the day after.  He did not restarted the server after installing these updates.  He did restarted the server on May 10th.  When the errors occurred he switched the job from using his account to the account used to run the sqlagent which now works.  This used to work before he applied SP1.  

Do you have an explanation why this happened in order to give a better explanation to the DBA.  Mind you I am not a SQL expert.