The Event ID is pretty generic, so you could get more information, performing a dtrace of the Archiving Task, while those 2243 Event IDs are appearing in your Enterprise Vault Applicaiton Log.
Within the Dtrace, an EV~E corresponds to an error and EV~W corresponds to a warning. So if you capture this error in the dtrace and then scroll up a bit, you might be able to determine why it's occurring. Make sure the thread number is consistent - they'll be enclosed in a <>
For example, if you are look at three lines in a trace, the line in the middle, thread 1002, is not part of the thread that generated the error, 1000 is the thread we focus on.
<1000> xxxxxxxxx
<1002> xxxxxxxx
<1000> EV~E xxxxx
Since it's Ex5.5, make sure that the Vault Admin account has the appropriate permissions at the Site and Configuration Level.
Is this error being generated for all of the Exchange Mailboxes on that Exchange 5.5 Server or just a few? If all, it may be a permissions issue, a problem with the EV system mailbox for that Exchane 5.5 server, or MAPI. If just a few, check for anything unusual about those mailboxes. For example, are they hidden? disabled? etc?