I'm not sure if this is all SEF, but when using acesef there was an issue around headers already sent.
As it turns out, the component doesn't route the form post result, causing SEF to break.
Fix:
In /components/com_ckforms/views/ckforms/view.html.php line 35 add a JRoute to $formLink. New line should read:
$this->assignRef( 'formLink',JRoute::_($formLink) )
Cheers,
Jason