Only works for admin

Hello!
Great tool! Many thanks for making super software.
I'm having an odd issue here: only administrators (NT-wise) can use the application. Any other user trying to access the folder is just asked for his username/pass again, and does not even get the WebExplorer lite logon screen. After 3 tries this error is shown:
HTTP Error 401.3 - Unauthorized: Access is denied due to an ACL set on the requested resource.
The users all can access both the folder with the app and the virtual dir with the files to be browsed. For any administrator account, it all works just fine. When I add a regular user to the administrators group, it works for that user as well
Web server: Windows 2003(SP1) / IIS6
Any idea's/suggestions welcome!

Some more info:
Got an intranet site where I want to use WebExplorer Lite to access a folder on the same server. I've got a folder 'FileBrowser' in the root of my site, containing the wexlite files. All intranet users have 'read' permission on this folder (tried with 'modify' permission, doesn't make a difference) I also made a virtual dir 'FileShare' which is linked to a (local) folder. All users have 'read' permission on this folder (again, has been tried with 'modify').
The users can log on to the intranet site successfully (NT-auth) and can browse to /FileShare which shows the contents of this folder (Windows/IIS 'browse' interface) When regular users browse to /FileBrowser, they experience the beforementioned problem.
Administrators can browse to /FileBrowser, and they get the wexlite interface, which works like a charm.
It does not seem to matter whether I enable of disable the wexlite password.
I've checked, all permissions are correctly applied to all (sub-)files and folders.
I'm using other ASP applications in the same site which do work for regular users, so I don't expect it to be an application pool problem or anything.
Has anyone else got a comparable setup running? Any ideas welcome!

Anyone?