I have the same error.
I have windows2000 running, all the latest service packs. I have dotnet 1.1 + 2.0, latest servicepacks. SQL Server 2000, latest patches. I downloaded DNN 2 days ago. I created a new database (DotNetNuke), gave dnn full rights to it, everyone has r/w access to c:\DotNetNuke. Webconfig and all is fine.
It installs fine, including all database migrations. Then it gives me the same error you got. I saw a few other forum threads about this as well but no answers yet.
One thing: ISS doesnt use 'Default' as default WWW root. I renamed it because I wanted to host multiple sites. I hope this doesnt matter too much.
<localhost>/DotNetNuke/Install.aspx?mode=none says everything is fine and has the latest version number.
The error I get when starting DotNetNuke is this one:
[NullReferenceException: Object reference not set to an instance of an object.]
DotNetNuke.Entities.Portals.PortalSettings.GetPortalSettings(Int32 TabId, PortalAliasInfo objPortalAliasInfo) +125
DotNetNuke.Entities.Portals.PortalSettings..ctor(Int32 tabId, PortalAliasInfo objPortalAliasInfo) +83
DotNetNuke.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e) +5768
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +92
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64
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Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.42
Any ideas?