I was having problems installing a clean version of DNN 040800 and thought it was at first maybe the installer wizard. I come to find out that the error i will post later happens when i actually add the AD Provider 1.0.0. Initially i had chosen to select it with a custom install. The site never even comes up. When i do a typical, it works just fine. I add the provider and same problem.

Server Error in '/Intranet' Application.

---

Value cannot be null.
Parameter name: path1

[ArgumentNullException: Value cannot be null.
Parameter name: path1]
   System.IO.Path.Combine(String path1, String path2) +2686595
   DotNetNuke.UI.Skins.SkinDefaults..ctor(SkinDefaultType DefaultType) +127
   DotNetNuke.Common.Globals.get_DefaultSkin() +31
   DotNetNuke.Entities.Portals.PortalSettings.GetPortalSettings(Int32 TabId, PortalAliasInfo objPortalAliasInfo) +2812
   DotNetNuke.Common.Globals.GetHostPortalSettings() +216
   DotNetNuke.Common.Globals.GetPortalSettings() +79
   DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule.Init(HttpApplication application) +213
   System.Web.HttpApplication.InitModulesCommon() +66
   System.Web.HttpApplication.InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) +814
   System.Web.HttpApplicationFactory.GetNormalApplicationInstance(HttpContext context) +243
   System.Web.HttpApplicationFactory.GetApplicationInstance(HttpContext context) +106
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +340

I would really appreciate any help I can get on this.

It didnt show up immediately. It showed that the provider had succesfully installed. I got the error as soon as I clicked return.

I was just able to at least get the site working again by commenting out in web.config the last key:

    <httpModules>
      <!-- add name="Authentication" type="DotNetNuke.HttpModules.AuthenticationModule, DotNetNuke.HttpModules.Authentication" / -->
      <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
      <add name="Compression" type="DotNetNuke.HttpModules.Compression.CompressionModule, DotNetNuke.HttpModules" />
      <add name="RequestFilter" type="DotNetNuke.HttpModules.RequestFilter.RequestFilterModule, DotNetNuke.HttpModules" />
      <add name="UrlRewrite" type="DotNetNuke.HttpModules.UrlRewriteModule, DotNetNuke.HttpModules" />
      <add name="Exception" type="DotNetNuke.HttpModules.Exceptions.ExceptionModule, DotNetNuke.HttpModules" />
      <add name="UsersOnline" type="DotNetNuke.HttpModules.UsersOnline.UsersOnlineModule, DotNetNuke.HttpModules" />
      <add name="DNNMembership" type="DotNetNuke.HttpModules.Membership.MembershipModule, DotNetNuke.HttpModules" />
      <add name="Personalization" type="DotNetNuke.HttpModules.Personalization.PersonalizationModule, DotNetNuke.HttpModules" />
      <!-- <add name="Authentication" type="DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule, DotNetNuke.Authentication.ActiveDirectory" /> -->
    </httpModules>

I think I've found what's causing the problem and have logged it in Gemini http://support.dotnetnuke.com/Default.aspx?p=23&i=6901. However this means that you can't use the AD Provider with DNN 4.8.0.