Is this supposed to happen with folder permissions in ASP.NET?

Hi there,

created a folder on my webserver and dropped some aspx files in them.
No problem, they do exactly what I want when I look at them in my
browser.

Then I added full control for a local user I created (let's call him
CLIENTUSER) to this folder and the files below. This user is member of
the local group USERS (as per default). So far, so good.

Then I removed permissions for the local Users group, forcing
CLIENTUSER to be confronted with a dialog box, right? Logging in seems
to work fine, but then I get the Runtime error saying I should have
custom errors=Off in web.config (which I have by the way) etc. etc.

Restoring RWX rights for the local Users groups solves the problems,
but also removes the popping up of the dialog box of course.

How can I set these permissions and NOT have the error?

Thanks

Cees