It looks like this is what XenForo uses to generate that error message:
PHP:
$previousVersion= $this->_getTemplateModel()->getLatestTemplateHistoryForTemplate(
$template['title'], $parentTemplate['style_id'], $template['last_edit_date']
);
if (!$previousVersion)
{
return $this->responseError(new XenForo_Phrase('no_previous_version_of_parent_could_be_found'));
}
So my guess is that there is no longer a template history stored for the template you're trying to edit, most likely caused by your 'Daily Clean Up' cron that prunes template edit histories. Unfortunately this is completely out of our control.