</configSections>
  <connectionStrings>
    <add name="SiteSqlServer" connectionString="Server=xxxxxxxxx.com;Database=xxxxxxxx;uid=xxxxxx;pwd=xxxxxx;" />
  </connectionStrings>
  <appSettings>
    <!-- Connection String for SQL Server 2005 Express - kept for backwards compatability - legacy modules  
    <add key="SiteSqlServer" value="Data Source=.\SQLExpress;Integrated Security=True;User Instance=True;AttachDBFilename=|DataDirectory|Database.mdf;"/>
    -->
    <!-- Connection String for SQL Server 2000/2005 - kept for backwards compatability - legacy modules -->
    <add key="SiteSqlServer" value="Server=xxxxxxxx.com;Database=xxxxxxx;uid=xxxxxxx;pwd=xxxxxxxx;" />
    <add key="InstallTemplate" value="DotNetNuke.install.config" />
    <!--  Alternative Install Templates (included in package)
    <add key="InstallTemplate" value="Club.install.config" />
    <add key="InstallTemplate" value="Personal.install.config" />
    <add key="InstallTemplate" value="SmallBusiness.install.config" />
    -->
    <add key="AutoUpgrade" value="true" />
    <add key="InstallMemberRole" value="true" />
    <add key="ShowMissingKeys" value="false" />
    <add key="EnableWebFarmSupport" value="false" />
    <add key="EnableCachePersistence" value="false" />
    <add key="InstallationDate" value="3/15/2006" />
  </appSettings>

I had the same error, after an hour of making the same alterations to the web.config I went for a cup of tea. Came back to the computer and realised what I had done! 

I was playing about installing DNN on a hosted server and had the usuall fun in getting all the web.config settings sorted.  In uploading the web.config I had accidently copied web.config to the route of the site.

eg.
 www.mysite.co.uk  (accidental web.config)
www.mysite.co.uk/dnn (where I was getting the error from)

Hope this helps someone else!

I had the same issue.  Wasted alot of time trying to figure it out.  The documentation isn't really clear on this topic but the resolution is very simple. As previously stated, you need to reference one datasource or another.   Simply, but carefully, comment out one of them. 

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