problems with new articles in exchange IMAP folders

[Steinar Bang <sb@metis.no>]

Platform: XEmacs 20.4, linus, Gnus v5.8.8 (CVS checkout from July 26 2000)

Starting two days ago, every time I get a new article in my Exchange
IMAP INBOX, I get the message 
	Wrong type argument: number-char-or-marker-p, FW:
(or something similar.  I've seen "RE:" and parts of the sender
address in place of "FW:") in the minibuffer, and a beep.

I can fetch the message from the INBOX using fetchmail.  The resulting 
message in an nnml folder looks normal, and with this message gone I
can read the old messages in the IMAP INBOX folder.

However the next time a message enters the INBOX the same problem
occurs. 

Are there any closer diagnostics I can run to figure out what is going 
on?

The full trace from C-h l from the attempt at opening this INBOX is:
	Wrong type argument: number-char-or-marker-p, FW:
	Fetching headers for nnimap+neptun:INBOX...done
	Fetching headers for nnimap+neptun:INBOX...
	nnimap: Updating info for nnimap+neptun:INBOX...done
	nnimap: Updating info for nnimap+neptun:INBOX...
	Retrieving newsgroup: nnimap+neptun:INBOX...