"Cannot open this item" error message

I'm running Office 2007, on XP. I periodically get the following error when
I try to open a message in Outlook:

"Cannot open this item. 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."

I get this error from time to time. Problems:
- I am the server administrator, and I have not set any such limits.
- This error can occur when I have zero items open (no e-mails,
appointments, tasks or anything). I can see where you might want to limit
the number of open items, but I'm pretty sure that number should be something
greater than "one".
- There are no open or new messages for me to close or un-attach.

Closing and restarting Outlook fixes it, but I'm looking for an actual
solution.

Thanks!

--
Denis