You know the text for this event is:
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In most cases, this behaviour is caused by some corrupt messages within the PST.
When a PST fails to migrate using the Enterprise Vault PST migration tool, you should first run the Outlook feature Inbox Repair Tool called scanpst.exe. This can diagnose and repair errors in the file if you can't open your Personal Folders file (.pst) or Offline Folder file (.ost) file, or you suspect that your .pst or .ost data file is corrupt
scanpst.exe scans only the .pst or. ost file, making sure that the file structure is intact. It doesn't scan your mailbox on the Microsoft Exchange server.
scanpst.exe is installed when you install Microsoft Outlook and is located at:
drive:\Program Files\Common Files\System\Mapi\1033\
For detailed information about running the tool, see the Help file, scanpst.hlp, at the same location.
Note: Use scanost.exe, the OST integrity check tool, if you get error messages when you synchronize your .ost file with the Exchange server.
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Are you sure this isn't corrupt PSTs and the V6 upgrade is just a coincidence?