I just wanted to add that I VERY regularly experience this and it's quite frustrating. I have XAML files located in many different resource folders, and the only way I temporarily resolve this is manually right clicking and "reimporting" the contents of the directory. But seemingly randomly things revert back after some time and I have to repeat this process.

It makes it nearly unusable when you're spending 30% of your dev time just reimporting XAML.

Are those XAMLs properly grabbed by the currently active scene?

In my case they were yes.

In my case, I am programmatically assigning assets to the NoesisView. The result is that it fails to show anything related to WPF, but the rest of my scene still operates (i.e. not a hard crash).

If I right click in the resource folder that has my xaml/assets and select "re-import", then I can visually see the asset get recreated and have an icon that shows a preview. Then pressing play again seems to work. (then randomly at some point in the future it fails again)

Since I have these in different resource folders throughout my Unity project, I essentially have to chase all of these down when it occurs.

If you are programmatically assigning assets to NoesisView make sure those assets are hard-referenced by Unity, by having them in the chain of dependencies of the scene. For example, adding each one to a public property of a MonoBehaviour.

We have plans to better report these issues, when a XAML not correctly referenced is being loaded.

But that's a different topic regarding this issue.