Imapsync: NO APPEND error saving message

I am using imapsync 1.525 to migrate mail into zarafa 7.1.4 on ubuntu 12.04. For quite some messages (about 32000) I get the following error:

    Sent 7021 bytes
    Read:   219 NO APPEND error saving message
    ERROR: 219 NO APPEND error saving message at /usr/share/perl5/Mail/IMAPClient.pm
    line 1363
    Mail::IMAPClient::_get_response('Mail::IMAPClient=HASH(0x225e3b0)', 219,
    undef) called at /usr/share/perl5/Mail/IMAPClient.pm line 1310
    Mail::IMAPClient::_imap_command_do('Mail::IMAPClient=HASH(0x225e3b0)',
    'HASH(0x22c1eb8)', 'APPEND "INBOX/My Folder " () "25-Aug-2010
    14:58:31...') called at /usr/share/perl5/Mail/IMAPClient.pm line 1209
    Mail::IMAPClient::_imap_command('Mail::IMAPClient=HASH(0x225e3b0)',
    'HASH(0x22c1eb8)', 'APPEND "INBOX/My Folder " () "25-Aug-2010
    14:58:31...') called at /usr/share/perl5/Mail/IMAPClient.pm line 2920
    Mail::IMAPClient::append_string('Mail::IMAPClient=HASH(0x225e3b0)',
    'INBOX/My Folder ', 'Return-Path:
    <sender@senderdomain.com>\x{d}\x{a}Delivery-Date: Wed, 25 Au...', '', '"25-Aug-2010
    14:58:31 +0000"') called at /root/imapsync-1.525/imapsync line 3009
    main::copy_message(2, 'My Folder ', 'INBOX/My Folder ', 'HASH(0x2292128)', '\Seen \Draft \Deleted \Flagged \Answered
    $Forwarded', '/tmp/imapsync_cache/imap.acme.de/my.name@mydomain.com/l...')
    called at /root/imapsync-1.525/imapsync line 1490
    ERROR: 219 NO APPEND error saving message at /usr/share/perl5/Mail/IMAPClient.pm
    line 1258
    Mail::IMAPClient::_imap_command('Mail::IMAPClient=HASH(0x225e3b0)',
    'HASH(0x22c1eb8)', 'APPEND "INBOX/My Folder " () "25-Aug-2010
    14:58:31...') called at /usr/share/perl5/Mail/IMAPClient.pm line 2920
    Mail::IMAPClient::append_string('Mail::IMAPClient=HASH(0x225e3b0)',
    'INBOX/My Folder ', 'Return-Path:
    <sender@senderdomain.com>\x{d}\x{a}Delivery-Date: Wed, 25 Au...', '', '"25-Aug-2010
    14:58:31 +0000"') called at /root/imapsync-1.525/imapsync line 3009
    main::copy_message(2, 'My Folder ', 'INBOX/My Folder ', 'HASH(0x2292128)', '\Seen \Draft \Deleted \Flagged \Answered
    $Forwarded', '/tmp/imapsync_cache/imap.acme.de/my.name@mydomain.com/l...')
    called at /root/imapsync-1.525/imapsync line 1490
    - msg My Folder /2 {7019} couldn't append
    (Subject:[=?iso-8859-1?Q?AW=3A_my_subject?=])
    to folder INBOX/My Folder : 219 NO APPEND error saving message
    msg My Folder /3 copying to INBOX/My Folder 
    internal date from host1: [29-Aug-2010 22:17:42 +0000]
    internal date from host1: ["29-Aug-2010 22:17:42 +0000"] (fixed)
    msg My Folder /3 date ["29-Aug-2010 22:17:42 +0000"] flags [] size
    [1341]

Other messages have been migrated fine. The attachment directory has permissions for zarafa, and is abut 150G large, so I assume zarafa is successfully storing attachments there. Also there is enough space on the filesystem.

Zarafa gateway tells me (loglevel 6):

    Thu Apr  4 13:44:30 2013: [28997] < 219 APPEND "INBOX/My Folder " ()
    "25-Aug-2010 14:5
    8:31 +0000" {7019}
    Thu Apr  4 13:44:30 2013: [28997] > + Ready for literal data
    Thu Apr  4 13:44:30 2013: [28997] < <7019 bytes data> Return-Path: 

    <REMOVED MAIL DATA FOR PRIVACY>

    Thu Apr  4 13:44:30 2013: [28997] Trying to parse alternative multipart 1 of
    mail body
    Thu Apr  4 13:44:30 2013: [28997] > 219 NO APPEND error saving message

It seems strange that the folde name terminates with a whitespace ( "INBOX/My Folder " ), but that seems not to be a problem, other messages have been migrated successfully to that folder.

I wondered about the "$Forwarded" flag, which seems odd as all the other flags start with a backslash. But in a test account this flag migrated fine.

Any ideas on how to solve this?