iis 6 - Asp.Net 4 Form Authentication issue -

i've strange issue .net 4.0 website published on iis6.

it implements forms authentication, , in web.config i've like:

<authentication mode="forms">   <forms loginurl="~/login.aspx" timeout="2880" name="auth_token" /> </authentication> <authorization>     <allow roles="__authuser__" />     <deny users="*" /> </authorization> 

and i've default.aspx setted default document in website.

if try access www.site/default.aspx redirects me www.site/login.aspx?returnurl=%2fdefault.aspx

and fine.

the problem if try access www.site/default.aspx?returnurl= gave me 401.2 error page: not authorized.

it's happening rebuild of intranet application , many users (for reason) have www.site/default.aspx?returnurl= url saved in bookmarks... need work.

any ideas?