Could you try the following workaround to try to get the megamenu working?
1. In /modules/mod_sclogin/helper.php, add the following before the first function __construct line
public static $modalOutput;
2. In /modules/mod_sclogin/tmpl/login.php around line 86 could you replace
<?php echo HTMLHelper::_('bootstrap.renderModal', $modalId, $modalParams, $modalContents) ; ?>with
[code[<?php
$val = HTMLHelper::_('bootstrap.renderModal', $modalId, $modalParams, $modalContents) ;
if (!$helper::$modalOutput)
$helper::$modalOutput = $val;
echo $helper::$modalOutput;
?>[/code]
If this works, I would suggest making a template override of the login file. I don't plan on checking this into the main sclogin codebase.
If you care about the source of why it's not working - It appears that the helix ultimate template megamenu is attempting to load the module multiple times. Somehow it and Joomla return the correct module contents the first time the module is loaded (with the login-modal div), but the subsequent times, the modal contents are not present correctly in the HTML.
Let me know how it goes.
-Melissa