Well, we already knew that. Without more information on the server configuration, here’s a temporary workaround, and it won’t help if this rears its head in other places in the Manager.
The page that is displayed when you get the login form is in /manager/media/style/MODxLight, and it’s named login.html.
Go into the folder that you unzipped on your local machine and find that file. Open it in a text editor, and change this line, line 194:
<form method="post" name="loginfrm" id="loginfrm" action="processors/login.processor.php">
Change the action="processors/login.processor.php" to "/manager/processors/login.processor.php". Then upload the file into your site’s manager/media/style/MODxLight folder. That should at least get you in to the manager.
Obviously something is adding the path to the login.html file to that relative path. Hopefully changing that to an absolute path (starting with /, which is your web root) will solve the immediate problem.