What's up with this? I've scoured the forums and have found no solution. DNN 5.1.2 on Windows Server 2003, fully patched, .NET 3.5 SP1, etc. SQL Server 2008.

• DNN 5.0.1 was running just fine
• Upgraded to 5.1.2
• Installer reports success at every step
• Login fails from: 
  • IE8 on WinXP
  • IE6 on Server 2003
  • IE7 on Server 2003
  • IE8 on Server 2008
• Login succeeds from:
  • Firefox 3.5 on Server 2003
  • IE8, Firefox 3.5, Safari 4 and Google Chrome on Windows 7

Here's the error from the DNN Event Log

AssemblyVersion: 5.1.2
PortalID: 0
UserID: -1
UserName:
ActiveTabID: 38
ActiveTabName: Home
RawURL: /Home/tabid/38/ctl/Login/Default.aspx?returnurl=%2fdefault.aspx
AbsoluteURL: /Default.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; WOW64; Trident/4.0; SLCC1; .NET CLR 2.0.50727; .NET CLR 3.0.30618; .NET CLR 3.5.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: a04bc340-4452-47fc-8d06-6cd57b626a09
InnerException: Unhandled Error:
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializePrimitiveObject
StackTrace:
Message: System.Exception: Unhandled Error: ---> DotNetNuke.Services.Exceptions.PageLoadException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentException: Invalid JSON primitive: . at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializePrimitiveObject() at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth) at System.Web.Script.Serialization.JavaScriptObjectDeserializer.BasicDeserialize(String input, Int32 depthLimit, JavaScriptSerializer serializer) at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit) at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize[T](String input) --- End of inner exception stack trace --- at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at DotNetNuke.UI.Utilities.Reflection.InvokeGenericMethod[T](Type Type, String MethodName, Object Target, Object[] Args) at DotNetNuke.UI.Utilities.MSAJAX.Deserialize[T](String Data) at DotNetNuke.UI.Utilities.ClientAPI.GetClientVariableList(Page objPage) at DotNetNuke.UI.Utilities.ClientAPI.GetClientVariableNameValuePair(Page objPage, String strVar) at DotNetNuke.UI.Utilities.ClientAPI.GetClientVariable(Page objPage, String strVar) at DotNetNuke.UI.WebControls.DNNMenu.LoadPostedJSON() at DotNetNuke.UI.WebControls.DNNMenu.OnInit(EventArgs e) at System.Web.UI.Control.InitRecursive(Control namingContainer) at System.Web.UI.Control.AddedControl(Control control, Int32 index) at System.Web.UI.ControlCollection.Add(Control child) at DotNetNuke.UI.Skins.NavObjectBase.InitializeNavControl(Control objParent, String strDefaultProvider) at DotNetNuke.UI.Skins.Controls.Nav.OnInit(EventArgs e) at System.Web.UI.Control.InitRecursive(Control namingContainer) at System.Web.UI.Control.InitRecursive(Control namingContainer) at System.Web.UI.Control.AddedControl(Control control, Int32 index) at System.Web.UI.ControlCollection.Add(Control child) at DotNetNuke.Framework.DefaultPage.Page_Init(Object sender, EventArgs e) 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 --- at DotNetNuke.Services.Exceptions.Exceptions.ProcessPageLoadException(Exception exc, String URL) at DotNetNuke.Framework.PageBase.OnError(EventArgs e) at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.default_aspx.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) --- End of inner exception stack trace ---