Repeating event id 8207 on Exch2003 backend server

[zaccaz]

Hi there, exchange 2003 sp2 native mode running on w2k3 sp1 server frontend & backend server, on the backend exchange server, since mar09 it got repeating event id 8207 for about every 90 mins, details as follows:

Event Type: Error
Event Source: MSExchangeFBPublish
Event Category: General
Event ID: 8207
Date: 12/5/2009
Time: 7:23:51 PM
User: N/A
Computer: BACKEND
Description:
Error updating public folder with free-busy information on virtual machine BACKEND. The error number is 0x80004005.
For more information, click

http://www.microsoft.com/contentredirect.asp.

I don't have any other corresponding event id 8197 or 8276, just only 8207

already rebooted server but didn't help, however this error didn't affect send/recv emails

any comments on how to fix it? many thx!