Do you have multiple Domino servers configured that the Notes Databases can fail over to? Do all of the servers have the Domino Mailbox Archiver agent loaded?
Two things I would need to look further into this:
1) Notes.ini from the Domino server(s). Specifically want to see what is listed in the extmgr section
2) A log file is generated in the domino data directory - CVLnDMHook.log. Could you reply and attach a copy of that log file?