"Error: Your server administrator has limited the number of items you can open simultaneously" (4294429)

Migration log file shows the following:

15:51:22 WARNING: [4951-24-120-80040305]Unable to migrate message:
15:51:22  Msg: (OFF5A84E9D:8A4BF592-ON85257A9F:004B108B)
15:51:22  Folder: (~INBOX)
15:51:22  Subject: (Performance Goals)
15:51:22  Sender: ("Jane Doe" <Jane.Doe@example.org>)
15:51:22  Date: (Mon Oct 22 09:40:42 2012)
15:51:22  Original message: notes:///85256DDA00745A31//F5A84E9D8A4BF59285257A9F004B108B
15:51:22 Unable to set message body.
15:51:22 RTFSync:
15:51:22 MAPI error 80040305
15:51:22 Error: Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing.
15:51:22 Component: Microsoft Exchange Information Store

Event Viewer on the Exchange Server configured as the CAS in the Exchange Server settings for MNE also shows the following: