Users reported a problem with Windows and the imapsync release 1.219
To fix the problem try this :

Near line 1170 there are 2 lines :

#unless($new_id = $to->append_string($t_fold,$string, $flags_f, $d)){
unless($new_id = $to->append_file2($t_fold, $message_file, "", $flags_f, $d)){

The first is commented with a # character, the second is not.
Remove the # on the first line and add a # at the beginning
of the second line. Run imapsync again and tell me if
your problem is solved.

This bug is fixed in revision 1.231