Error when manually archiving or restoring items: "Could not process the selected items. Reason: Enterprise Vault is currently unavailable. Try again later."

Problem

Manual archiving and restoring items with Outlook fails with the error below, while restoring items from a web browser (Search or Archive Explorer) works normally.

Error Message

The following error is shown in Outlook:

Could not process the selected items. Reason: Enterprise Vault is currently unavailable. Try again later.

The Client Trace will show the following: 

09/04/2013 13:29:51.548[1676]: HTTP request error: 366 (503)
09/04/2013 13:29:51.549[1676]: HTTP request error. Enterprise Vault is currently unavailable. Try again later. 503
09/04/2013 13:29:51.549[1676]: ~CHTTPRequest::CheckStatus
09/04/2013 13:29:51.549[1676]: ~CHTTPRequest::Connect
09/04/2013 13:29:51.549[1676]: CHTTPRequest::Disconnect
09/04/2013 13:29:51.549[1676]: ~CHTTPRequest::Disconnect
09/04/2013 13:29:51.550[1676]: Connect failed: 0x80004005
09/04/2013 13:29:51.550[1676]: CHTTPRequest::RequestFailed: 0x0
09/04/2013 13:29:51.550[1676]: BackendHTTPImpl::MarkedItems<&public: long __thiscall CItemMarker::UndoMarkArchiveMe(void)>::Unmark: 0x0
09/04/2013 13:29:51.553[1676]: CItemMarker::UndoMarkArchiveMe: 0x0
09/04/2013 13:29:51.556[1676]: Undo marking for archive:

Cause

There are multiple causes for this error, and several possible solutions are listed below.

Solution

Solution 1:

The error could result from the Vault Service Account's (VSA) credentials being incorrect. Reset the Vault Service Account (VSA) password at the Directory level of the Enterprise Vault Administration Console. Detailed instructions are in this related article.

Solution 2:

The error can be seen if the archiving tasks are in the process of restarting. The frequency with which they restart automatically can be controlled via the registry setting: RestartAllMAPITaskIntervalMins. 

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\KVS\Enterprise Vault\Agents (64 bit OS)

Changes to this setting will take effect after the Enterprise Vault Admin Service restarts. 

Solution 3:

The error can result from the failure of a mailbox archiving task. Confirm all archive tasks are in a 'running' state so that they are able to respond to archiving and restoring requests.

Solution 4: GPO is pushing the following two registry keys to the servers:  They must be removed if they exist, which can only be done by modifying the GPO and allowing replication to occur or a reboot.

Solution 5:

The error can be caused by insufficient permissions on the Enterprise Vault installation directory. Check that both the Authenticated Users and IIS_IUSRS groups are not denied access to the installation directory. By default, this directory is C:\Program Files (x86)\Enterprise Vault. 

Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.