New Post: Manual upgrade 4.0 -> 4.1

Deleting the table from the database fixed it for me.

However, I ran into another problem on pages where I use WebForms Validator Controls:

 Exception type: InvalidOperationException 
    Exception message: WebForms UnobtrusiveValidationMode requires a ScriptResourceMapping for 'jquery'. Please add a ScriptResourceMapping named jquery(case-sensitive).
   bei System.Web.UI.ClientScriptManager.EnsureJqueryRegistered()
   bei System.Web.UI.WebControls.BaseValidator.RegisterUnobtrusiveScript()
   bei System.Web.UI.WebControls.BaseValidator.OnPreRender(EventArgs e)
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Control.PreRenderRecursiveInternal()
   bei System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Searching for this error got me to this SO question: http://stackoverflow.com/questions/16660900/webforms-unobtrusivevalidationmode-requires-a-scriptresourcemapping-for-jquery

I followed the advice there and put

<appSettings>
      <add key="ValidationSettings:UnobtrusiveValidationMode" value="None" />
    </appSettings>

in the web.config, which fixes it.

Apparently "None" is not the default value for targetFramework="4.5", which can lead to problems. Hope this helps other people with a similar problem.