I have deleted the message as suggested but it did not work, the errors keep coming. The file that stalls is always 25,6 MB in size???
What is the maximum file upload size on your server? Are you really syncing ZD with a Zimbra 4.5.6 server? If that's the case I'd suggest as a matter of priority you get yourself onto a recent version of Zimbra - there have been several important security updates since that version.