I'm sorry I don't have the details, but....
There are a few articles on deleting the local mail file and forcing it to be rebuilt. This has worked in a few cases here. (Running Exchange 5.5) Try a google search on "Cached Mode problems".
Typically, it involves turning it off, exiting Outlook, turn it back on, restart Outlook. In some cases, you may need to actually delete the local copy.
I've run into enough problems using cached mode w/ Exchange 5.5, that we turn it off by default. (Normal default is on).