Do you have an SEF extension installed on your site? The only difference between the SCLogin's logout button and Joomla's is that our redirection URL is including your domain name. The Joomla logout url is using a relative link. That difference shouldn't matter, but it looks to be the cause.
To fix, please edit the /modules/mod_sclogin/helper.php file. Around line 120, you'll see:
if (!$url)
{
$uri = JURI::getInstance();
$url = $uri->toString(array('scheme', 'host', 'path', 'query'));
}Update the 2nd to last line so it looks like:
$url = $uri->toString(array('path', 'query'));Please test, again, and let us know how that goes.
Thanks,
Alex