Rules run manually but not automatically

[eczematous]

If I select my rules to run automatically, then every time I receive
any email, it stays at the top of my Inbox and I get the error message:

"The folder you are trying to move or copy the message to can't be
found. Make sure the rule refers to a valid folder."

And yet, if I go to Run Rules Now and apply all of the same rules
manually to what has just arrived, they work fine, as they always had
until yesterday.

I'm using Outlook 2003 (11.6568.6568) SP2.