Discovery mailbox can't be found

"The discovery mailbox, a hidden default mailbox that is required to search mailboxes, can’t be found. It may have been inadvertently deleted. This mailbox must be re-created before you can search mailboxes.”

If you are getting the above error when trying to run Audit reports from ECP such as Run a non-owner mailbox report try the following suggestions.

1. If the mailbox SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9} has not been deleted and is still in the users OU try enabling it by running the powershell command.  This is all I had to do to get it working. After I did my Exchagne 2007 to Exchange 2010 Migration.

Enable-Mailbox  -Identity "SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}" -Arbitration

2. If Discovery System Mailbox has been deleted then you will need to recreate it (Will not stop mail flow and will require no downtime)

• Make sure the Mailbox SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9} is not present if it is you can delete it.
• Install the Exchange media into a Domain Controller and run Setup /PrepareAD
• Enable the mailbox using EMC - Enable-Mailbox -Identity "SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}" -Arbitration

This should solve your issue.

http://technet.microsoft.com/en-us/library/gg588318.aspx