Problem synchronizing local Sent folder with server

[Thomas Weber <thomas.weber.mail@xxxxxxxxx>]

Hi,

version 5.99.4, Debian unstable package

I have new messages in my local Sent folder. However, offlineimap barks
when trying to synchronize this with the imap server:

Server (out of my control):
        Cyrus IMAP4 v2.1.18-IPv6-Debian-2.1.18-1+sarge2

The end of the log:
==========================================================================
MainThread: imap: savemessage: new content length is 1327
MainThread: imap:   07:38.99 > JEK21 APPEND "INBOX/Sent" (\Seen) "
6-Feb-2008 07:08:49 +0100" {1327}
MainThread: imap:   09:18.99 < JEK21 NO System I/O error
MainThread: imap:   09:18.99    matched r'(?P<tag>JEK\d+) (?P<type>[A-Z]+)
(?P<data>.*)' => ('JEK21', 'NO', 'System I/O error')
MainThread: imap:   09:18.99 NO response: System I/O error
==========================================================================

I'm rather clueless where to look.

Thanks
        Thomas