In my case this behaviour was triggered through a missing line in my modified qa-theme function:
function q_view_main($q_view){....}
I had to change
if (isset($q_view['main_form_tags']))
$this->output('</FORM>');
to
if (isset($q_view['main_form_tags'])) {
$this->form_hidden_elements(@$q_view['buttons_form_hidden']);
$this->output('</form>');
}
Just for info...
EDIT
For all voting, buttons and favorite forms which one has in his custom qa-theme.php , the closing part needs to be reviewed, as in 1.6.1 these forms are closed differently like the example above, which is for buttons only