Commenting out the dependency property and turning the property setter/getter to no-ops resolves the behavior. I'm investigating further...

Things I tried to prevent crash that does not work
1) comment out all code in constructor, including loaded event.
2) make get/set no-op.
3) rename to ScalingEnabledProperty to ScalingEnabled2Property.
4) use different constructor for dependency property to avoid using PropertyMetadata(true) parameter. Result: crash occurs in DependencyPropertyExtend, Line 13 instead.

Things I tried that does prevent the crash
1) comment out the dependency property, ScalingEnabledProperty. See works.cs for what code looks like that does work every time.

What is strange is that this worked fine for several days (2+ weeks?) and now it is happening every time I load/start Unity. My only workaround right now is to remove the DependencyProperty ScalingEnabledProperty.

using Noesis;
using System.Collections.Generic;
using UnityEngine;
using Grid = Noesis.Grid;
using Vector2 = UnityEngine.Vector2;

namespace StellarConquest.Presentation.Unity.UI
{
    public partial class ViewboxGridControl : Grid
    {
        //public static readonly DependencyProperty ScalingEnabledProperty = DependencyProperty.Register("ScalingEnabled", typeof(bool), typeof(ViewboxGridControl), new PropertyMetadata(true));

        public static readonly List<Vector2> AspectRatios = new List<Vector2>()
        {
            new Vector2(7680, 4320), // 0
            new Vector2(3840, 2160), // 1
            new Vector2(2560, 1440), // 2
            new Vector2(1920, 1080), // 3
            new Vector2(1600, 900),  // 4
            new Vector2(1366, 768),  // 5
            new Vector2(1280, 720),  // 6
            new Vector2(1152, 648),  // 7 
            new Vector2(1024, 576),  // 8
        };

        public static int DefaultAspectRatioIndex = 5;

        public ViewboxGridControl()
        {
            Loaded += (sender, e) =>
            {
                if (PlayerPreferences.Instance != null)
                    SetScale(PlayerPreferences.Instance.UserInterfaceSize);
                else
                    SetScale(DefaultAspectRatioIndex);
            };
        }

        public void SetScale(int index)
        {
            if (ScalingEnabled)
            {
                Width = AspectRatios[index].x;
                Height = AspectRatios[index].y;
            }
        }

        public Vector2 GetViewboxScale()
        {
            Viewbox viewbox = this.Parent as Viewbox;
            if (viewbox == null)
                return new Vector2(1, 1);
            float x = viewbox.ActualWidth / ActualWidth;
            float y = viewbox.ActualHeight / ActualHeight;

            return new Vector2(x, y);
        }

        public bool ScalingEnabled
        {
            get
            {
                //return (bool)GetValue(ScalingEnabledProperty);
                return false;
            }
            set
            {
                //SetValue(ScalingEnabledProperty, value);
            }
        }
    }
}

Maybe bool needs to be added as a valid property type?

It seems a problem with the loading of Noesis library while starting Unity.

Could you please try the following code for your ViewboxGridControl?

using Noesis;

using System.Collections.Generic;

using Vector2 = UnityEngine.Vector2;

using Grid = Noesis.Grid;

namespace StellarConquest.Presentation.Unity.UI

{

public partial class ViewboxGridControl : Grid

{

static ViewboxGridControl()

{

Noesis.GUI.Init();
        ScalingEnabledProperty = DependencyProperty.Register("ScalingEnabled", typeof(bool), typeof(ViewboxGridControl), new PropertyMetadata(true));
    }

    public static readonly DependencyProperty ScalingEnabledProperty = null; // DependencyProperty.Register("ScalingEnabled", typeof(bool), typeof(ViewboxGridControl), new PropertyMetadata(true));
    ...
}
}

OK, perfect. Your analysis is spot on. The change worked, and helped me identify the cause.

The class contains a static property, ViewBoxGridControl.DefaultAspectRatioIndex. MonoBehavior code referred to this variable which, when accessed, caused the static initializer for ViewBoxGridControl to be called, which includes initialization of the dependency property. I moved the constant to a better location and all is well. Candidate for closure -- thank you for helping me understand this.

Thanks for the feedback stonstad.

I wonder if we could detect this scenario sergio, like having asserts if Noesis API is trying to be used without initialization. Not sure how many entries and how many asserts would be needed. And we don't want this code to be present in the non-editor build.

Yes, we should be more robust against this scenario.
I'll move this ticket for the next release so we can think about how to solve it.