Still trying to figure out what is causing these. It seems related to mirroring. I've got DDL logging on the server and I see lots of CREATE_EVENT_NOTIFICATION events happening, all by NT Authority / System (which is not the account the SQL Server or Agent services are running under). The only SQL-related services running under the Local System account are SQL Server VSS Writer and SQL Full-test Filter Daemon Launcher.
My DDL logging shows lots of these events being created, but only some of them are failing and showing up in the Windows Event log. The majority seem to be created (and later dropped) just fine. In fact, going by the GUIDs, all the ones showing in my DDL logging work (and don't show in the event log) and the ones in the event log I do not see in the DDL logs (which makes sense, if they are failing).
These events start happening during a maintenance plan that pauses mirroring, disables some WMI alert I have set up for mirroring state changes, performs a DBCC CHECKDB and some index defrags, then re-enables mirroring and the alerts. In the Database Mirroring Monitor, I have no warnings enabled.
Mirroring seems to be working OK. Databases are synchronized. There are no messages in the SQL logs related to this.
I have no idea what is causing these errors to be written to the event log. Can anyone help?