I just did a new installation of DNN v5.1.0 and all I get when I try to view the new site is the following error in my URL field:

...Default.aspx?tabid=39&error=Object+reference+not+set+to+an+instance+of+an+object.&content=0

When I look in the SQL database table, "EventLog," I get the following:

<LogProperties>
<LogProperty>
<PropertyName>AssemblyVersion</PropertyName>
<PropertyValue>5.1.0</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>PortalID</PropertyName>
<PropertyValue>0</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>PortalName</PropertyName>
<PropertyValue>Johnson Ferry Baptist Church</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>UserID</PropertyName>
<PropertyValue>-1</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>UserName</PropertyName>
<PropertyValue />
</LogProperty>
<LogProperty>
<PropertyName>ActiveTabID</PropertyName>
<PropertyValue>39</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>ActiveTabName</PropertyName>
<PropertyValue>Home</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>RawURL</PropertyName>
<PropertyValue>/Default.aspx?tabid=39&amp;error=Object+reference+not+set+to+an+instance+of+an+object.&amp;content=0</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>AbsoluteURL</PropertyName>
<PropertyValue>/Default.aspx</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>AbsoluteURLReferrer</PropertyName>
<PropertyValue />
</LogProperty>
<LogProperty>
<PropertyName>UserAgent</PropertyName>
<PropertyValue>Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.0.11) Gecko/2009060215 Firefox/3.0.11</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>DefaultDataProvider</PropertyName>
<PropertyValue>DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>ExceptionGUID</PropertyName>
<PropertyValue>f7e63542-ad00-4d5b-ad75-276ff2361b91</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>InnerException</PropertyName>
<PropertyValue>Object reference not set to an instance of an object.</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>FileName</PropertyName>
<PropertyValue />
</LogProperty>
<LogProperty>
<PropertyName>FileLineNumber</PropertyName>
<PropertyValue>0</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>FileColumnNumber</PropertyName>
<PropertyValue>0</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>Method</PropertyName>
<PropertyValue>DotNetNuke.UI.Skins.Skin.GetSkin</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>StackTrace</PropertyName>
<PropertyValue />
</LogProperty>
<LogProperty>
<PropertyName>Message</PropertyName>
<PropertyValue>DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---&gt; System.NullReferenceException: Object reference not set to an instance of an object.
   at DotNetNuke.UI.Skins.Skin.GetSkin(PageBase Page)
   at DotNetNuke.Framework.DefaultPage.Page_Init(Object sender, EventArgs e) in D:\Inetpub\wwwroot\JFBC2009\Website\Default.aspx.vb:line 480
   at System.Web.UI.Control.OnInit(EventArgs e)
   at System.Web.UI.Page.OnInit(EventArgs e)
   at DotNetNuke.Framework.PageBase.OnInit(EventArgs e)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   --- End of inner exception stack trace ---</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>Source</PropertyName>
<PropertyValue />
</LogProperty>
</LogProperties>

The problem seems to have something to do with the Skin, but I cannot figure out what. I have tried restarting IIS to see if that would clear it up and it did not. The _default skin is in place, but there are no other skin or container directories.

The installation went through without a problem. Does anyone have an idea what I should try?

This error happens to two of my websites, at least once per day. Both sites were installed from DNN version 5.2.2 from scratch (not an upgrade from a previous version.)  When the problem occurs the website is dead and on the generic DotNetNuke error appears but the logs show the error:

Object reference not set to an instance of an object. at DotNetNuke.UI.Skins.Skin.GetSkin(PageBase Page) at DotNetNuke.Framework.DefaultPage.Page_Init(Object sender, EventArgs e) at System.Web.UI.Control.OnInit ...

The only way I have found to fix it is to copy all the DLL's back to the bin folder using FTP. This resets the web application and causes a recompile and clears out whatever the problem was. It's good for another 12 hours or so before it happens again.  Of course, this is not going to work as a long-term solution and I'm hoping that someone will come up with a permanent solution.

I've seen this too. It's because the core API method GetSkin was changed - and this breaks older code that used the previous version of the method.  It would have been easy to keep the old API method and add a new one.  I don't know why this breaking change was allowed - usually DNN is pretty good about backwards compatibility.

Oscar