Compare the number of the controls folders, 6.5.XXXX.X between the front End and the back End (they need to have the same highest number! Ensure that this highest number is supported (see the compatability matrix).
C:\Program Files\Exchsvr\exchweb
If they are all the same, then look to the proxycfg Technotes.
Type proxycfg at the command prompt on the Exchange Server
Does it have the Exchange Server's IP address listed first?
Is the Enterprise Vault Server listed correctly in all its various forms? (e.g. NetBios name, Alias, FQDN, etc?)