apache mod_rewrite fails when file by same name exists

The complete .htaccessfile

RewriteEngine on
RewriteBase /

RewriteRule ^([a-z0-9_\-]+)/([a-z0-9_\-]+)/(index\.php)?$ /main.php?type=$1&action=$2 [NC,QSA,L]

so,

www.server.com/users/new/

redirects to:

www.server.com/main.php?type=users&action=new

The rule works fine on my test machine.

However,
on another machine, it fails if there is a file that starts with the same name:

/
|- main.php
|
+--[users]
    |- new.inc.js

requesting

www.server.com/users/list/       --> 200 OK (main.php returns results as expected)
www.server.com/users/new/        --> 404 Not Found

If I remove `new.inc.js' the request will be redirected as expected.

What configuration flag causes this behaviour?
(I've been banging my head against the wall for some time now)

Update (I added the complete .htaccess file contents) index.html and index.php are the directory indexes

(And it does not make sense to me either :()