Comments I found when searching on the net for reasons.

Comments Massimo Luinetti (Last update 5/12/2007):
If you have performed an upgrade from version 2003 to version 2007 and you have only one configuration database, you will probably have to disable the old “SharePoint Portal Administration Service” (version 2003). That was the solution in my case.

Michel Dawidowicz (Last update 1/28/2005):
In my case, this symptom started to appear when the database backend was down for a short period. The event is logged twice per minute in the log. To resolve this restart the “Sharepoint Portal Administration” service on each affected server of the farm.

Ionut Marin (Last update 11/3/2004):
From a newsgroup post: “Go to IIS manager, open each site, and go to the home directory tab. Then, there is a section at the bottom called Application pools. Edit each and set the account to someone who has logon as a service right to the local server”.