"WARNING: [4951-24-120-80040305] Unable to migrate message:" (4212200)

During the Calendar migration, you have seen the following Warning in the log file:

06:21:12 WARNING: [4951-24-120-80040305] Unable to migrate message:
06:21:12 Msg: (OF74036FD1:9B7355A4-ON9A8EB2EB:AAA6EF22)
06:21:12 Folder: (~CALENDAR)
06:21:12 Subject: (Subject)
06:21:12 Sender: ("FirstName LastName" <firstname.lastname@domain.com>)
06:21:12 Date: (Wed Apr 13 11:15:11 2011)
06:21:12 Original message: notes:///85256A8100466182//74036FD19B7355A49A8EB2EBAAA6EF22
06:21:12 Unable to set message body.
06:21:12 RTFSync:
06:21:12 MAPI error 80040305
06:21:12 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.
06:21:12 Component: Microsoft Exchange Information Store
06:21:12 Low level error: 000004DE
06:21:12 Context: 0000054A