Did you try using the /resetfolders switch?
If you are getting this warning for the user it means that the folder ID for Memos has changed. In this users mailbox every folder has an ID and they should never change.
The BES will only sync the defualt folders, Contacts/Calendar/Notes/Tasks. Right now to the BES the Memos folder looks like it was deleted and re-created and the BES will not sync this folder any more because of that.
There is an article from RIM on how to check the folder ID to see if it did indeed change:
KB11639 - EA process stops when attempting to synchronize the memo or contact list