Last Updated: June, 23, 2026
ISSUE
Users may experience sync issues in classic Outlook where email folders are not automatically updated.
Trying to manually update a folder through Send/Receive > Update folder results in the following error:
“Task ‘ ‘reported error (0x80004005) : The operation failed.’”
Capturing a Fiddler network trace will show the following service exception:
System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.Exchange.Data.CopyOnWrite.M365BackupHelper.IsFolderDeletedItems(IStoreSession session, StoreObjectId folderId)
STATUS: FIXED
The service team started the rollout of the fix which will be available in server builds >15.21.0155.000.
While the rollout is ongoing you can leverage the column "Version" in the Outlook Connection Status to review which build is running in the server your mailbox is hosted.
If this is a high impact scenario on your organization and your users are still running below the fixed build, please contact Microsoft Support.
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See Also
Fixes or workarounds for recent issues in classic Outlook for Windows
Fixes or workarounds for recent issues in new Outlook for Windows