Resetting the important mark

Hi,

I have two messages which I once gave a high importance (the "$" sign)
and now I am unable to take that back again. After refetching the
message (C-u .) I can take away the mark but it comes back after
leaving and revisiting the folder.

The error message when I try to unmark is:

elmo-imap4-accept-ok: IMAP error: The specified message set is invalid.
IMAP error: The specified message set is invalid. [2 times]

Is there anything I could do?

Thank you very much
Petrik