The following event is logged in the Application log on the SharePoint Server 2010 front-end Web server
when you perform a search:
Additionally, you receive less relevant search results when you perform a search.
This issue occurs because the Search Query Logging thread reuses expired credentials.
To resolve this issue, apply this following hotfix package:
2459257 Description of the SharePoint Server 2010 Cumulative Update Server Hotfix Package (MOSS server-package): December 31, 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.