If you view the \Sync Issues folder in Microsoft Outlook, you find a synchronization log with an entry similar to the following:

17:27:28 Synchronizer Version 14.0.4760
17:27:28 Synchronizing Mailbox 'user name'
17:27:28 Synchronizing Hierarchy
17:27:44 Synchronizing Forms
17:27:44 Downloading from server 'server.corp.com'
17:27:45 Error synchronizing folder
17:27:45 [80004005-501-4B9-560]
17:27:45 The client operation failed.
17:27:45 Microsoft Exchange Information Store
17:27:45 For more information on this failure, click the URL below:
17:27:45 http://www.microsoft.com/support/prodredirect/outlook2000_us.asp?err=80004005-501-4b9-560
17:27:45 Done

The 4B9 portion of the error code translates to ecNullObject from Store, indicating that the failure occurred because an object that was referenced was null.  This could be due to number of resons including, but not limited to, a permissions issue or because the Outlook client is not able to access the public folder replica.

Possible Resolutions:

Resolution 1:  Problems with the Replica

If there are multiple Exchange servers in the environment there could be a possible replication issue with the EFORMS REGISTRY system folder, removing the replica and adding it back has resolved the issue in some cases.

Use the following commands to remove and re-add the replica:

1. To check which Public Folder Database the Exchange Mailbox is using, run the following cmdlet in Exchange Management Shell:
   
   get-MailboxDatabase |fl Name, PublicFolderDatabase
   
   

   Example:
   Name                        :  MailboxDatabase1
   Public Folder Database:  Public Folder Database 1

   Name                        :  MailboxDatabase2
   Public Folder Database:  Public Folder Database 2 <-- Users on this mailbox database are getting the sync errors

   Name                        :  MailboxDatabase3
   Public Folder Database:  Public Folder Database 1

2. To get a list of the replicas for the EFORMS REGISTRY run the following cmdlet:
   
   get-publicfolder -Identity "\NON_IPM_SUBTREE\EFORMS REGISTRY" -Recurse | fl Name,Replicas

   Example:
   Name      : EX:/o=contoso/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)
   Replicas : {Public Folder Database 1}

   Name      : EX:/o=contoso/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)
   Replicas : {Public Folder Database 2}

    

3. Remove the replica from the database you found in step #1.  This is most easily accomplished using the Public Folder Managment Console from Exchange Management Console (EMC):

   1. In the console tree, click Toolbox.

   2. In the result pane, double-click Public Folder Management Console.

   3. In the console tree of the Public Folder Management Console, expand System Public Folders, and then expand the EFORMS REGISTRY public folder and click on Organizational Forms Library.
   4. In the result pane, click the public folder you want, and then, in the action pane, click Properties.

   5. Use the Replication tab to view the and set the content replication schedule and to specify the databases on which the public folder content will be replicated.

      • Under Replicate content to these public folder databases select a database from the list

      • Click the X to remove it from the replication list.

4. Re-Add the replica from the database you found in step #1 using the same Public Folder Management Console:
   
   

   1. In the console tree, click Toolbox.

   2. In the result pane, double-click Public Folder Management Console.

   3. In the console tree of the Public Folder Management Console, expand System Public Folders, and then expand the EFORMS REGISTRY public folder and click on Organizational Forms Library.

   4. In the result pane, click the public folder you want, and then, in the action pane, click Properties.

   5. Use the Replication tab to view the and set the content replication schedule and to specify the databases on which the public folder content will be replicated.

      • Under Replicate content to these public folder databases click the Add button to open the Select Public Folder Database dialog box.  Use this dialog box to specify which public folder databases this public folder should be replicated to. 

      • Select the public folder database, and then click OK
         

Resolution 2:  Problems with the Permissions

The error can be caused by incorrect permissions on the EFORMS REGISTRY for the Default user.

1. To check permissions on the EFORMS REGISTRY, run the following cmdlet in the Exchange Management Shell:
   
   Get-PublicFolderClientPermission “\NON_IPM_SUBTREE\EFORMS REGISTRY” | fl user, accessrights

Example:
User           : Default
AccessRights: {Reviewer}

User           : Anonymous
AccessRights: {None}

1. In the output, examine the permissions given to the Default user.  They should not be set to None.  The following is a list of accepted permissions for the Default user: 
   
   - Owner
   - PublishingEditor
   - Editor
   - PublishingAuthor
   - Author
   - NonEditingAuthor
   - Reviewer
   - Contributor
   
2. If the Default user has it's permissions set to None, run the following command to set the permissions to Reviewer. 
   
   Add-PublicFolderClientPermission “\NON_IPM_SUBTREE\EFORMS REGISTRY” -User Default -AccessRights Reviewer

For more information about the Set-PublicFolder command, visit the following Microsoft Web site:
http://technet.microsoft.com/en-us/library/aa998596.aspx

For more information about the Add-PublicFolderClientPermission command, visit the following Microsoft Web site:
http://technet.microsoft.com/en-us/library/bb124743.aspx

Configure Public Folder Properties
http://technet.microsoft.com/en-us/library/bb691327.aspx