Ok, trying this again.
My game (UWP) works in Debug Mode, both on Windows 10 and the Xbox One.
It works in Release mode, but only if I turn off the ".NET Native tool chain" compile option in the project properties.
It fails in Release mode with that same option turned on. The code compiles, but fails at runtime.
However, that option is needed to create an application package that will pass validation.
The part of my game that fails in Release mode is when I'm trying to load content from an XML file:
Effects.SmokeBomb = Globals.ContentManager.Load<ProjectMercury.ParticleEffect>("Particles\\smokebomb");
This fails with an exception at runtime: System.IndexOutOfRangeException
Again, this line compiles and runs just fine in DEBUG and RELEASE (no .Net native toolchain) mode.
It appears to be related to the issue reported here: http://community.monogame.net/t/custom-content-pipeline-serialization-issues-in-windows-10/7665/3
... particularly the last post in the thread.
I'm pretty new here, but does this make sense to anyone?