You just need to take $active[0] instead of $active, these changes work for me:

 

1/ Edit the administrator/components/com_k2/models/extrafield.php file
Line 208:
Replace :
$output='<textarea name="K2ExtraField_'.$extraField->id.'" id="K2ExtraField_'.$extraField->id.'" rows="10" cols="40" class="k2ExtraFieldEditor">'.$active[0].'</textarea>';
By :
$editor =& JFactory::getEditor();
$output = $editor->display("K2ExtraField_".$extraField->id."", $active[0], '550', '400', '60', '20', false);

2/ Edit the administrator/components/com_k2/models/item.php file
Line 511:
Replace :
$variables = JRequest::get('post');
By :
$variables = JRequest::get('post',JREQUEST_ALLOWRAW);