Unfortunately I had no luck with this approach, Scott. I created a new folder under "Cabinet" and moved my recent messages there, out of the Inbox. Mac Mail can see that new folder and all of the messages in it, but still can't access the Inbox.
I did find this page
which suggests that the folder structure may have been changed. From my end in webmail and Mac Mail, it doesn't look like my Mailbox has moved. Any chance this could be checked from the server side?