Did you look at this technote?
Error when retrieving an archived item in Outlook after migrating the EV server to Windows Server 2008 R2
Enable 32-bit mode on the EnterpriseVaultAppPool ...
It seems like something went wrong with the database upgrade. Run the EVDatabaseUpgrader.exe located in the Enterprise Vault installation directory. Also, don't forget to backup all the Enterprise Vault databases before running this too...
Take a look at this technote:
Using Exchange Server database availability groups
To ensure that the mailboxes you enable for archiving are always available to Enterprise Vault, you must set up...
As GertjanA mentioned, use EVSVR to verify the savesets in the Vault Store and to report any missing or corrupted data. Take a look at this technote for the steps:
How to use EVSVR to verify savesets in the vault store database