We are not suggesting to turn off SEF URLs permanently. Most websites, including this one, use Joomla with SEF URLs. It's simply a test.
On the login page, I can see the following body tag:
<body class="site-default navigation-default theme-default layout-home nav-breakpoint-xl navigation-hide com_users view-login item-636"
That is showing that com_users view-login is likely being loaded, but it's not viewable on the page. As mentioned above, com_users is not something we control.
The next -test- I'd recommend is to -temporarily- switch to the default Joomla template and go to the same page. If the Joomla login page shows, that indicates your template is altering the com_users output and hiding it for some reason.
Please test that and let us know if the login form (and possibly JFBConnect's social buttons) show up.
Thanks,
Alex