IIS 7.5 / ASP.net 4.5 - Windows authentication works 50%

Hi,

I'm writing a small application which should facilitate helpdesk tasks - user creation for instance is part of that.

I'm facing a strange issue with windows authentication. When I'm starting the web-page as user who has administrative permissions I'm authenticated without prompting and can access Active Directory.

BUT: When I'm launching the website as a helpdesk staff, got prompted for authentication, after entering credentials the app recognizes me - form-validation doesn't work and I can't read information from Active Directory.

My config:
+  IIS 7.5 running on Windows 2008 R2, fully patched
+ .net framework 4.5.1
+ powershell 4

Modifiied aspnet.config file:
 <legacyImpersonationPolicy enabled="false"/>
 <alwaysFlowImpersonationPolicy enabled="true"/>

Extended Token buffer for IIS:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters]
"MaxRequestBytes"=dword:16777216
"MaxFieldLength"=dword:00065534


Application Pool:
- Managed Pipeline: Integrated
- Identity: ApplicationPoolIdentiy

Authentication:
- ASP.NET Impersonation: Enabled
- Windows Authentication: Enabled
-- Enabled Providers: Negotiate:Keberos
-- Enable Kernel-mode authentication (not checked)

Bindings:
- pocs:99
- pocs.nwtraders.com:99

NTFS security (of the folders containing my app)
- SYSTEM: Full Control
- Administrators: Full Control
- Users: Read
- IIS AppPool\pocs: Read

Computer Object(of the computer object where IIS is running)
SPN
- HTTP:pocs
- HTTP:pocs.nwtraders.com

Delegation:
- Trust this computer for delegation to any service (Kerberos only)

web.config (of app)
<configuration>
 <appSettings>
     <add key="CKeysFile" value="App_Data/CKeys.txt" />    
   </appSettings>
 <system.web>
  <customErrors mode="Off" />
  <compilation debug="true" targetFramework="4.5.1">
   <assemblies>
    <add assembly="System.Management.Automation, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <add assembly="System.DirectoryServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
   </assemblies>
  </compilation>
  <httpRuntime targetFramework="4.5.1" />
        <identity impersonate="true" />
  <authentication mode="Windows" />
 </system.web>
 <system.webServer>
        <validation validateIntegratedModeConfiguration="false" />
    </system.webServer>
</configuration>

Many thanks in advance


Ruben