APPEND command error

[David J Byler]

I'm attempting to sync two IMAP servers (multiple accounts) using
offlineimap.  Things were going quite well with some accounts syncing
perfectly, and then more and more accounts started erroring out with
errors similar to this:

ERROR: Saving msg folder 'FOLDERNAME', repo 'REPOSITORYNAME'failed
(error). Server reponded: APPEND command error: BAD ['Command Argument
Error. 11']. Data: XXXXX APPEND FOLDERNAME (\Seen) "07-Jul-2011 08:14:57
+0000" {58453083}

where I've substituted FOLDERNAME and REPOSITORYNAME for the real folder
and repository names, and XXXXX for an apparently random 5 or 6 digit
string...

Any thoughts?  As far as I can tell it's having trouble updating the
"seen" flag on the destination repository.

I started out with offlineimap 6.5.5-rc2 and also tried 6.5.4.