I got this error because I had turned on all "Common Language Runtime Exceptions" in the "Exception Settings" window of Visual Studio.
This must be one of the exceptions that fires as part of the normal flow of ASP.NET. If that's your problem (or if you don't know what the "Exception Settings" window does and you want to reset it to default) you should probably open the "Exception Settings" window and click the "Restore the list to the default settings" button at the top, or at least uncheck the exceptions you don't want the debugger to break on.
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