Does this happen every time on every computer/browser, or just sometimes, or just on a specific computer ?

Have you enabled cache expiration on the files ?

Does the problem go away when you hit CTRL-F5 ?

Does this happen every time on every computer/browser, or just sometimes, or just on a specific computer ?

Everytime and we have another intrasite too (without demotemplates), there is same problem.

Have you enabled cache expiration on the files ?

Do you meen web.config file settings? PageFiles, Documents and Global setting are:
<system.web>
      <httpHandlers>
        <add path="WebResource.axd" verb="GET" type="System.Web.Handlers.AssemblyResourceLoader" validate="true" />
        <add path="*" verb="*" type="EPiServer.Web.StaticFileHandler, EPiServer" validate="true" />
      </httpHandlers>
    </system.web>
    <staticFile expirationTime="-1.0:0:0" /> 

Does the problem go away when you hit CTRL-F5 ?

Nope..

Weird, I suggest that you use FireBug or Fiddler and look at the actual HTTP traffic when this problem happen.

Yes i have been looking for this also in Fiddler. There is nothing strange, when user lost access to file or picture system rise loginscreen. Path to login.aspx is also lost, http error 404, weird ?

http://testintranet/login.aspx?ReturnUrl=%2fDocuments%2ftestidoc.doc 

The resource cannot be found.

Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable.  Please review the following URL and make sure that it is spelled correctly.

Requested URL: /login.aspx

I save couple of fiddler sessions to file, can i send you e-mail?

ops, there was error in web.config, comment closing tag was wrong place and close out login.aspx.

<authentication mode="Windows">
    <!-- <authentication mode="Forms"> -->
      <forms name=".EPiServerLogin" loginUrl="Util/login.aspx" timeout="120" />
 </authentication>

Problem still exits, now user got loginscreen, ofcourse.

I'm out of ideas, if you think its an impersonation problem you should be able to spot that with FileMon on the server and look for access denied.

we have the same problem in a intranet project

however: it works on two out of three servers. but on one the behaviour is just as you describe above!

I have tried reinstalling episerver and moved the config from the other servers but no success!

Is it your intention to use Windows authentication?

Does a clean install with the public or demo-templates work with Windows authentication or do you get the same issue?

IIS6 or IIS7?

We have windows authentication becouse its intranet, we use IIS6 and clean public and demo installs.

Try following this FAQ when you set up windows authentication:

http://world.episerver.com/FAQ/Items/How-to-set-up-automatic-logon-with-windows-authentication/

I found a way to make it work for us.

 the solution was to sett the authentication in the metabase to "Negotiate,NTLM" - and now it works fine.

 problem was most likely impersonation issues - i got the clue from the eventlog...

We tried that and it's works!  Thank you all!  

Try following this FAQ when you set up windows authentication:

http://world.episerver.com/FAQ/Items/How-to-set-up-automatic-logon-with-windows-authentication/