for me this helped: (7.0.9)
in inc/utils.inc.php, line 592
$oConfig->set('HTML.DefinitionID', '1');
//$oDef = $oConfig->getHTMLDefinition(true);
//$oDef->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');
if ($oDef = $oConfig->maybeGetRawHTMLDefinition()) {
$oDef->addAttribute('a', 'target', 'Enum#_blank,_self,_target,_top');
}