VB error - The Call Stack Contains Only External Code

I've programmed a small application in visual studio 2012 using VB.NET 3.5 framework. It was running perfectly when I last modified the project, and published it. However today when I go in and try to run the code I get an error (see pic/below). I don't know what's causing this. Nothing has changed in the project since it was last published, this just seems to have suddenly happened!

Can anyone explain what's happened, and how to resolve it?

The Call Stack Contains Only External Code
>      mscorlib.dll!System.Runtime.Hosting.ApplicationActivator.CreateInstance(System.ActivationContext activationContext, string[] activationCustomData) + 0xe0 bytes      
       mscorlib.dll!System.Runtime.Hosting.ApplicationActivator.CreateInstance(System.ActivationContext activationContext) + 0xa bytes      
       mscorlib.dll!System.Activator.CreateInstance(System.ActivationContext activationContext) + 0x3e bytes      
       Microsoft.VisualStudio.HostingProcess.Utilities.dll!Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssemblyDebugInZone() + 0x23 bytes      
       mscorlib.dll!System.Threading.ThreadHelper.ThreadStart_Context(object state) + 0x66 bytes      
       mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x6f bytes      
       mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() + 0x44 bytes
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Fernando SotoConnect With a Mentor RetiredCommented:
You can try another thing first.
In the Visual Studio IDE click on the main menu "Build" and select from the context menu "Clean Solution". Then re-build the solution and run.

If that did not help look at the screen shot you posted in your question at the bottom you show a stack trace, just above that is a check box with the label Disable Just My Code, place a check mark on it then try again.
Fernando SotoRetiredCommented:
Have you tried placing a check mark when you get the dialog to Disable Just My Code and see what happens?
bjblackmoreAuthor Commented:
I'm not entirely sure how to do that? Can you provide details and I will give it a try. Like I said, last time I ran the app build/public it was fine, and I've got a working build. But for some reason, just going into yesterday and running debug it's giving that error. Nothing should have changed/been added to the code.
I don't know what this 'ApplicationActivator' or 'ActivationContext' is trying to do. I don't have any kind of application activation in the code. It's just a small 200kb app for updating user details in AD.
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bjblackmoreAuthor Commented:
There has to be something outside the code that's causing this error, as I just copied & pasted the code & form into a totally new project, and it's working. I could just keep the new project, but I'd love to know what caused the old one to throw up that error, in case it happens again!
bjblackmoreAuthor Commented:
I went through all of the application settings, and restored them to default, and checked against the new clean project, there was 1 extra reference (system.threading) which I removed and also did the clean solution. Seems to have fixed it, not sure exactly which of those, but at least it's working now!
Many thanks
Fernando SotoRetiredCommented:
Not a problem, glad to help.
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