Error Message

"The custom form could not be opened. Outlook will use an Outlook form instead. The form required to view this message cannot be displayed. Contact your administrator"

OR

Solutions

Solution 1:

To resolve this, the forms cache can be cleared or the forms can be deleted. 
 

In Outlook 2010, follow the steps below:  

1. File | Options | Advanced | Custom Forms | Manage Forms
    
2. Click the Clear Cache button
    
3. Close and Re-open Outlook and try again

Solution 2:

1. Delete the Enterprise Vault forms listed under Personal Forms and close Outlook
    
2. On the EV server, open the Vault Administration Console
    
3. Expand Policies | Exchange | Desktop and go to the Properties
    
4. Select the Advanced tab, then in the drop-down menu, choose Outlook
    
5. Confirm the setting Deploy Forms Locally is set to Always
    
6. Synchronize the mailbox after changing the policy settings.
    
7. Open the shortcut

Solution 3:

Unload the Antivirus Agent and test the behavior of the retrievals.

If the issue resolves by unloading the AV agent, then do the following.

1. Collect Procmon logs while reproducing the issue to find out the folders and files that are scanned by AV and contact the Anti-Virus Vendor to review the Anti-Virus Exclusions.

2. In this scenario, the following location must be excluded from the AV scan.

C:\Users\Username\AppData\Local\Microsoft\FORMS\FRMCACHE.DAT 

3. Any IPM.NOTE.EnterpriseVault.* subfolders should also be excluded from the AV scan.

By default, the Enterprise Vault Outlook forms are stored in the Personal Forms library.  In some instances, these forms are either not downloaded properly or have some underlying problem.