My workplace has Office 365 for Outlook and Rackspace for email hosting. I recently changed my Outlook from POP3 to IMAP. I also have IMAP on ios mail and Outlook ios apps. When the setup was done,most of the previous mails came into the new folder. But sending and receiving became very slow, with always "synchronizing subscribed folders" running. Now I'm not receiving most new emails to the desktop Outlook, but they are visible in mobile apps and Rackspace webmail. Outlook is also taking a lot of time to close.
From this error, it seems to be related with subscribed folders. To check, please first click Folder > Update Folder List. And then choose IMAP folders > Query.
Make sure all the foldes has been subscribed.
(You will notice 2 tabs above the list of folders, "All" and "Subscribed". The Goal is to make sure both Tabs match and have the folders you want to sync listed.)
After setting, restart Outlook to check the results.