I upgraded from 4.9.2 last night to 5.1.1. Everything seemed to go fine. No errors with install. I even used my portal for about an hour... upgraded to form and List module, etc.  Everything seemed to work adequately except the register link on the account login module (logged in gemini).

Now, this morning, the site menu (DNN:Nav) doesn't load at all, and the home page says:

A critical error has occurred.
Exception has been thrown by the target of an invocation. Shows up on my site.

 When I try to login from account login module, I get the same error or the "DotNetNuke logo" saying an error has occured.

I tried to reupload web.config to restart IIS, but it still doesn't work.

Any suggestions before I revert to 4.9.2?

Thanks Alex.

There are no third party modules on the main page at all.... I recently installed Ultra Video Gallery and it's up to date, and supposed to work with DNN 5.  I can't even login with ANY account.

When I uploaded a new skin.ascx and skin.css that uses SolPart Menu... the menu partially works.

No, I'm not  using a third party login module, it's from core.

Not using Godaddy, using 3essentials.

Event Log table from SQL database just shows PAGE_LOAD_EXCEPTIONS.

Here are some details:

Exception has been thrown by the target of an invocation. ---&gt; System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---&gt; 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&amp; 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 System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters) 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 ---</

Thanks Alex --

I did try that earlier... still didn't work with default skin.

I rebooted several times.... still didn't work.

Then, I had to go back into the office this evening and it suddenly started working without changing anything.

I'm wondering if it's the server now.... but my non-5.1.1 sites (4.9.2) never stopped working.

I'm on web22 at 3essentials.. .and apparently they've been having problems with DNN sites on web22 over the past few days.  Not sure if that's the problem, but it's just strange how it works off and on.