Debugging UWP .NET app: Unhandled exception at 0x79A1CFAC (SharedLibrary.dll) in MyApp.exe: 0x00001007

I've built a Xamarin.Forms app and when I try to debug its UWP frontend in Debug|AnyCPU mode I get this exception at launch:

Unhandled exception at 0x79A1CFAC (SharedLibrary.dll) in MyApp.exe: 0x00001007

Visual Studio wouldn't let me copy all the details to the clipboard so I had to take a screenshot:

If I run it in Debug|x64 mode I get a completely different exception:

Unhandled exception at 0x00007FFFEA232EC0 (Windows.UI.Xaml.dll) in MyApp.exe: 0xC000027B: An application-internal exception has occurred (parameters: 0x000002225D634960, 0x0000000000000002).

And this time the CallStack panel is empty.

I'm at a loss at how to approach this problem.