Outlook 2007: Sent Items Existing Folder selection blank?

I am using Outlook 2007 at work with three IMAP accounts. Through account settings, I am specifying that all sent emails go to the respective Sent Items folder of that account.

Outlook will randomly lose the ability to do this on one of the three accounts. It will default back to “Save sent mail in the Outlook Sent Items folder” and the existing folder window is blank. The New Folder button is greyed out.

Is there a limit or is this a bug? I can’t seem to find online answers anywhere. Any help would be greatly appreciated.