I’ve recently installed 'Senayan Library Management System (SLiMS) on my site. It all works, except some features. For example, I can’t add any authors. When I try to add these, all it shows is the Error ‘403 Forbidden’ page. Or at some other pages, all it says is ‘Unknown Error’. My theory is, that It can’t access these sites, because it’s embed with an Iframe (at least I think so) and something blocks it from accessing.
I don’t know, if it uses Iframe technology. And I also don’t know, what URL is supposed to be shown. But all it shows, is the standard ‘403 Forbidden’ site from infinity free. I also haven’t found an option to enable error messages.
Can you use your browser’s developer tools to see what URLs are redirecting to the 403 error page? I don’t know that software and I don’t have access to your admin panel, so I can’t check.