Event ID - 8208

Event Id8208
SourceEXCDO
DescriptionCalendaring agent failed with error code thread id while expanding recurring appointments.
Event InformationExplanation :
The Event 8207 indicates that the Calendaring Agent on the Exchange Server encountered an unexpected problem while processing a recurring appointment in the Calendar folder of the mailbox. This failure can occur for a variety of reasons and the error code in the Description section of the event will indicate the underlying cause of the failure to delete. For example, error 0xfffffa1d maps to JET_errColumnNotFound. Please use the ERROR.EXE program from the Exchange 2000 CD to get details on the error.
This error is seen if there is a corruption of the Calendar items in a mailbox or if the Calendar Folder on the mailbox is not accessible due to permissions issues or if there is a corruption in the database that the mailbox is homed on. Also, if the Exchange Installable File System (IFS) Drive (also called M Drive) is being scanned by a file level Virus Scanner (or has been scanned so in the past), then this error might be seen. 

User Action :
Ensure you have applied the latest Exchange 2000 Service pack. Service packs might include fixes for the EXCDO component (or other related components) and may fix the problem.

Re-register the following DLL files using the REGSVR32.EXE program: oledb32.dll, Oledb32r.dll, Msdaipp.dll, Msdaurl.dll and Msado15.dll.

Check to see if any kind of File Level Antivirus Scanner is scanning the M Drive. If so, configure the Antivirus software to exclude the M Drive (and also the Exchange Database and log directories) from a file level scan. You may have to reboot the server after making this change. Contact the Antivirus vendor for details. 

If the errors persist after the above changes, then you may have to run some maintenance utilities on the Exchange 2000 databases. Please contact Microsoft Product Support Services before performing this action.
Reference Links How to Troubleshoot the "503 Service Unavailable" Error Message in Outlook Web Access 2000

How to troubleshoot the "503 Service Unavailable" error message in Outlook Web Access in Exchange Server 2003

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