I got the same error with the install on my hosting environment.
My local windows installation worked fine.
The fengoffice/error_log doesn't show any critical errors. And turning on define('DEBUG', true); doesn't help either.
The AJAX error response is:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>403 Forbidden</title>
</head><body>
<h1>Forbidden</h1>
<p>You don't have permission to access /fengoffice/index.php
on this server.</p>
<p>Additionally, a 404 Not Found
error was encountered while trying to use an ErrorDocument to handle the request.</p>
</body></html>
Can anyone help please!