System.Windows.Markup.XamlParseException' occurred in PresentationFramework.dll?

This is not a problem with XAML. The error message is saying that it tried to create an instance of DVRClientInterface.MainWindow and your constructor threw an exception.

You will need to look at the "Inner Exception" property to determine the underlying cause. It could be quite literally anything, but should provide direction.

An example would be that if you are connecting to a database in the constructor for your window, and for some reason that database is unavailable, the inner exception may be a TimeoutException or a SqlException or any other exception thrown by your database code.

If you are throwing exceptions in static constructors, the exception could be generated from any class referenced by the MainWindow. Class initializers are also run, if any MainWindow fields are calling a method which may throw.

Problem solved! I'm using Ctrl + Alt + E to open Exception Window, and I checked all throw checkbox. So the debuger can stop at the exactly the error code.

It took me ages to work this one out, so for the benefit of searchers:

I had a bizarre issue whereby the application worked in debug, but gave the XamlParseException once released.

After fixing the x86/x64 issue as detailed by Katjoek, the issue remained.

The issue was that a CEF tutorial said to bring down System.Windows.Interactivity from NuGet (even thought it's in the Extensions section of references in .NET) and bringing down from NuGet sets specific version to true.

Once deployed, a different version of System.Windows.Interactivity was being packed by a different application.

It's refusal to use a different version of the dll caused the whole application to crash with XamlParseException.