I've got a debug dump (available at http://1drv.ms/1CpYkmr), which gives me very strange output. I tried doing standard WinDBG stuff and got nothing useful, so I tried PDE.dll and got the following:
=========================================================================================
PDE v9.5 - Copyright 2014 Andrew Richards
=========================================================================================
Start memory scan : 0x0552e634 ($csp)
End memory scan : 0x05530000 (User Stack Base)
0x0552e648 : 0x066dd578 : !dse combase!STOWED_EXCEPTION_INFORMATION_V1
0x0552e650 : 0x066dd578 : !dse combase!STOWED_EXCEPTION_INFORMATION_V1
0x0552e694 : 0x066dd578 : !dse combase!STOWED_EXCEPTION_INFORMATION_V1
0x0552e6ec : 0x066dd578 : !dse combase!STOWED_EXCEPTION_INFORMATION_V1
*** WARNING: Unable to verify checksum for System.Core.ni.dll
0:004> !PDE.dse
Stowed Exception Array @ 0x066dd578
Stowed Exception #1 @ 0x06728600
0x80004003 (FACILITY_NULL - Default): E_POINTER - Pointer that is not valid
Stack : 0xb9505d4
61e7f228 Windows_UI_Xaml!DirectUI::ButtonBase::ExecuteCommand+0x103
61e7f469 Windows_UI_Xaml!DirectUI::ButtonBase::OnClick+0xc9
621d87db Windows_UI_Xaml!DirectUI::Button::OnClick+0x9b
61e8033b Windows_UI_Xaml!DirectUI::ButtonBase::PerformPointerUpAction+0x60
61e80162 Windows_UI_Xaml!DirectUI::ButtonBase::OnPointerReleased+0x292
620290be Windows_UI_Xaml!DirectUI::ControlGenerated::OnPointerReleasedProtected+0x7e
61d5e28a Windows_UI_Xaml!DirectUI::Control::FireEvent+0x648bd5
617159e4 Windows_UI_Xaml!DirectUI::DXamlCore::FireEvent+0x2cf
617145c0 Windows_UI_Xaml!AgCoreCallbacks::FireEvent+0x40
617144f7 Windows_UI_Xaml!CCoreServices::CLR_FireEvent+0xe7
617143f5 Windows_UI_Xaml!CommonBrowserHost::CLR_FireEvent+0x35
617126b0 Windows_UI_Xaml!CControlBase::ScriptCallback+0xe0
61712a48 Windows_UI_Xaml!CXcpDispatcher::OnScriptCallback+0x21c
617124c5 Windows_UI_Xaml!CXcpDispatcher::WindowProc+0x186
75108e71 user32!_InternalCallWinProc+0x2b
751090d1 user32!UserCallWinProcCheckWow+0x18e
7510932c user32!DispatchClientMessage+0xdc
75109529 user32!__fnDWORD+0x49
77620996 ntdll!KiUserCallbackDispatcher+0x36
7510e4a9 user32!SendMessageW+0x139
61ace38b Windows_UI_Xaml!CXcpBrowserHost::SyncScriptCallbackRequest+0x11b
617159ba Windows_UI_Xaml!CEventManager::Raise+0x4ba
61acd396 Windows_UI_Xaml!CEventManager::RaiseRoutedEvent+0xd9
61f070c7 Windows_UI_Xaml!CInputManager::RaiseDelayedPointerUpEvent+0x21c
61f05419 Windows_UI_Xaml!CInputManager::ProcessGestureInput+0x279
61f06e63 Windows_UI_Xaml!CInputManager::ProcessTouchInteractionCallback+0xc3
61e2075b Windows_UI_Xaml!CCoreServices::ProcessTouchInteractionCallback+0x4b
61ec80f5 Windows_UI_Xaml!CUIElement::TouchInteractionCallback+0x29
61d8febe Windows_UI_Xaml!CTouchInteractionHelper::InteractionEngineCallback+0x5d5
61d8ffb4 Windows_UI_Xaml!CTouchInteractionHelper::StaticInteractionEngineCallback+0x14
6ae9a1ad ninput!COutputConverter::Process+0x1ec
6ae99fa0 ninput!CInteractionContextImpl::OutputCallback+0x7c
6ae99c09 ninput!CInteractionGroupingFilter::_SendOutput+0x29
6ae99d4c ninput!CInteractionGroupingFilter::Input+0x2d
6ae99cc5 ninput!COutputCoalescingFilter::Flush+0x49
6ae99c64 ninput!CInteractionEngineImpl::DigitizerInput+0x4e4
6ae97c21 ninput!ProcessInputInteraction+0x57
6ae99605 ninput!CInteractionContextImpl::ProcessFrameHistory+0x998
6aea08a4 ninput!ProcessPointerFramesInteractionContext+0x54
61a013bb Windows_UI_Xaml!CTouchInteractionHelper::ProcessPointerInformation+0x1e3
61a011c6 Windows_UI_Xaml!CTouchInteractionHelper::ProcessPointerMessage+0x66
61a01696 Windows_UI_Xaml!CInputManager::ProcessPointerMessagesWithInteractionEngine+0x160
61a05f08 Windows_UI_Xaml!CInputManager::ProcessInteractionPointerMessages+0x87
61a05e65 Windows_UI_Xaml!CInputManager::ProcessPointerInput+0x771
619ed303 Windows_UI_Xaml!CInputManager::ProcessInput+0xe5
61acd81e Windows_UI_Xaml!CCoreServices::ProcessInput+0x3e
61acd7a0 Windows_UI_Xaml!CXcpBrowserHost::HandleInputMessage+0x14a
619ed5c4 Windows_UI_Xaml!CJupiterControl::HandlePointerMessage+0x62
61bb7207 Windows_UI_Xaml!CJupiterControl::HandleWindowMessage+0x582fc3
6163410f Windows_UI_Xaml!CJupiterWindow::WindowProc+0x114
6163403b Windows_UI_Xaml!CJupiterWindow::StaticWindowProc+0x27
75108e71 user32!_InternalCallWinProc+0x2b
751090d1 user32!UserCallWinProcCheckWow+0x18e
7510a66f user32!DispatchMessageWorker+0x208
7510a6e0 user32!DispatchMessageW+0x10
6aee3b6d Windows_UI!Windows::UI::Core::CDispatcher::WaitAndProcessMessages+0x16f
6aee3c94 Windows_UI!Windows::UI::Core::CDispatcher::ProcessEvents+0x7a
617d1348 Windows_UI_Xaml!DirectUI::FrameworkView::Run+0x87
6b7b66ed twinapi_appcore!Windows::ApplicationModel::Core::CoreApplicationView::Run+0x3d
6b7b6505 twinapi_appcore!Windows::Foundation::Collections::Internal::HashMap<unsigned int,Windows::UI::Core::ICoreWindow *,Windows::Foundation::Collections::Internal::DefaultHash<unsigned int>,Windows::Foundation::Collections::Internal::DefaultEqualityPredicate<unsigned int>,Windows::Foundation::Collections::Internal::DefaultLifetimeTraits<unsigned int>,Windows::ApplicationModel::Core::Details::SmugglableInterfaceLifetimeTraits,Windows::Foundation::Collections::Internal::HashMapOptions<unsigned int,Windows::UI::Core::ICoreWindow *,Windows::Foundation::Collections::Internal::DefaultLifetimeTraits<unsigned int>,0,1,0> >::Remove+0x215
74674b16 SHCore!StringCchPrintfW+0x146
77197c04 kernel32!BaseThreadInitThunk+0x24
7763b54f ntdll!__RtlUserThreadStart+0x2f
7763b51a ntdll!_RtlUserThreadStart+0x1b
Stowed Exception #2 @ 0x06728628
0x80004005 (FACILITY_NULL - Default): E_FAIL - Unspecified failure
Stack : 0xb6d5dec
61d36064 Windows_UI_Xaml!CDependencyObject::SetEffectiveValue+0x429027
6190d5fb Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xd1
6190d579 Windows_UI_Xaml!CDependencyObject::SetValue+0x55
61716141 Windows_UI_Xaml!CUIElement::SetValue+0x61
61716288 Windows_UI_Xaml!CFrameworkElement::SetValue+0x78
6171cf47 Windows_UI_Xaml!CControl::SetValue+0x53
6171d00b Windows_UI_Xaml!CContentControl::SetValue+0x77
6194e9b0 Windows_UI_Xaml!CDependencyObject::SetThemeResourceBinding+0xba
6184d3f2 Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xb72
61867ff9 Windows_UI_Xaml!CFrameworkElement::OnStyleChanged+0x16b
6186834e Windows_UI_Xaml!CControl::ApplyBuiltInStyle+0x48
61895bee Windows_UI_Xaml!CControl::CreationComplete+0x7f
61895b63 Windows_UI_Xaml!XamlManagedRuntime::InitializationGuard+0x12c
6186220d Windows_UI_Xaml!XamlWriter::WriteNode+0x2e13
61871bc3 Windows_UI_Xaml!CParser::LoadXamlCore+0x404
618e31b0 Windows_UI_Xaml!CCoreServices::ParseXamlWithExistingFrameworkRoot+0xa0
618e301d Windows_UI_Xaml!CApplication::LoadComponent+0x21c
618e2dba Windows_UI_Xaml!Application_LoadComponent+0x95
618e2ccb Windows_UI_Xaml!DirectUI::Application::LoadComponent+0xbb
618e367d Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocationImpl+0x5d
618e3608 Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocation+0x28
Unable to load image Windows.UI.Xaml.ni.dll, Win32 error 0n2
*** WARNING: Unable to verify checksum for Windows.UI.Xaml.ni.dll
*** ERROR: Module load completed but symbols could not be loaded for Windows.UI.Xaml.ni.dll
6273101d Windows_UI_Xaml_ni+0x29101d
Stowed Exception #3 @ 0x06728650
0x80004005 (FACILITY_NULL - Default): E_FAIL - Unspecified failure
Stack : 0x7e5b6d4
61d36064 Windows_UI_Xaml!CDependencyObject::SetEffectiveValue+0x429027
6190d5fb Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xd1
6190d579 Windows_UI_Xaml!CDependencyObject::SetValue+0x55
61716141 Windows_UI_Xaml!CUIElement::SetValue+0x61
61716288 Windows_UI_Xaml!CFrameworkElement::SetValue+0x78
6171cf47 Windows_UI_Xaml!CControl::SetValue+0x53
6171d00b Windows_UI_Xaml!CContentControl::SetValue+0x77
6194e9b0 Windows_UI_Xaml!CDependencyObject::SetThemeResourceBinding+0xba
6184d3f2 Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xb72
61867ff9 Windows_UI_Xaml!CFrameworkElement::OnStyleChanged+0x16b
6186834e Windows_UI_Xaml!CControl::ApplyBuiltInStyle+0x48
61895bee Windows_UI_Xaml!CControl::CreationComplete+0x7f
61895b63 Windows_UI_Xaml!XamlManagedRuntime::InitializationGuard+0x12c
6186220d Windows_UI_Xaml!XamlWriter::WriteNode+0x2e13
61871bc3 Windows_UI_Xaml!CParser::LoadXamlCore+0x404
618e31b0 Windows_UI_Xaml!CCoreServices::ParseXamlWithExistingFrameworkRoot+0xa0
618e301d Windows_UI_Xaml!CApplication::LoadComponent+0x21c
618e2dba Windows_UI_Xaml!Application_LoadComponent+0x95
618e2ccb Windows_UI_Xaml!DirectUI::Application::LoadComponent+0xbb
618e367d Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocationImpl+0x5d
618e3608 Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocation+0x28
6273101d Windows_UI_Xaml_ni+0x29101d
Stowed Exception #4 @ 0x06728678
0x80004005 (FACILITY_NULL - Default): E_FAIL - Unspecified failure
Stack : 0x7e4fe5c
61d36064 Windows_UI_Xaml!CDependencyObject::SetEffectiveValue+0x429027
6190d5fb Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xd1
6190d579 Windows_UI_Xaml!CDependencyObject::SetValue+0x55
61716141 Windows_UI_Xaml!CUIElement::SetValue+0x61
61716288 Windows_UI_Xaml!CFrameworkElement::SetValue+0x78
6171cf47 Windows_UI_Xaml!CControl::SetValue+0x53
6171d00b Windows_UI_Xaml!CContentControl::SetValue+0x77
6194e9b0 Windows_UI_Xaml!CDependencyObject::SetThemeResourceBinding+0xba
6184d3f2 Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xb72
61867ff9 Windows_UI_Xaml!CFrameworkElement::OnStyleChanged+0x16b
6186834e Windows_UI_Xaml!CControl::ApplyBuiltInStyle+0x48
61895bee Windows_UI_Xaml!CControl::CreationComplete+0x7f
61895b63 Windows_UI_Xaml!XamlManagedRuntime::InitializationGuard+0x12c
6186220d Windows_UI_Xaml!XamlWriter::WriteNode+0x2e13
61871bc3 Windows_UI_Xaml!CParser::LoadXamlCore+0x404
618e31b0 Windows_UI_Xaml!CCoreServices::ParseXamlWithExistingFrameworkRoot+0xa0
618e301d Windows_UI_Xaml!CApplication::LoadComponent+0x21c
618e2dba Windows_UI_Xaml!Application_LoadComponent+0x95
618e2ccb Windows_UI_Xaml!DirectUI::Application::LoadComponent+0xbb
618e367d Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocationImpl+0x5d
618e3608 Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocation+0x28
6273101d Windows_UI_Xaml_ni+0x29101d
Stowed Exception #5 @ 0x067286a0
0x80004005 (FACILITY_NULL - Default): E_FAIL - Unspecified failure
Stack : 0x7dfa064
61d36064 Windows_UI_Xaml!CDependencyObject::SetEffectiveValue+0x429027
6190d5fb Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xd1
6190d579 Windows_UI_Xaml!CDependencyObject::SetValue+0x55
61716141 Windows_UI_Xaml!CUIElement::SetValue+0x61
61716288 Windows_UI_Xaml!CFrameworkElement::SetValue+0x78
6171cf47 Windows_UI_Xaml!CControl::SetValue+0x53
6171d00b Windows_UI_Xaml!CContentControl::SetValue+0x77
6194e9b0 Windows_UI_Xaml!CDependencyObject::SetThemeResourceBinding+0xba
6184d3f2 Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue+0xb72
61867ff9 Windows_UI_Xaml!CFrameworkElement::OnStyleChanged+0x16b
6186834e Windows_UI_Xaml!CControl::ApplyBuiltInStyle+0x48
61895bee Windows_UI_Xaml!CControl::CreationComplete+0x7f
61895b63 Windows_UI_Xaml!XamlManagedRuntime::InitializationGuard+0x12c
6186220d Windows_UI_Xaml!XamlWriter::WriteNode+0x2e13
61871bc3 Windows_UI_Xaml!CParser::LoadXamlCore+0x404
618e31b0 Windows_UI_Xaml!CCoreServices::ParseXamlWithExistingFrameworkRoot+0xa0
618e301d Windows_UI_Xaml!CApplication::LoadComponent+0x21c
618e2dba Windows_UI_Xaml!Application_LoadComponent+0x95
618e2ccb Windows_UI_Xaml!DirectUI::Application::LoadComponent+0xbb
618e367d Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocationImpl+0x5d
618e3608 Windows_UI_Xaml!DirectUI::ApplicationFactory::LoadComponentWithResourceLocation+0x28
6273101d Windows_UI_Xaml_ni+0x29101d
I might be interpreting the stacktrace wrong, but it looks to me like it's failing inside Windows.UI.Xaml when trying to call ExecuteCommand. It doesn't look like it's my actual command that's failing, but not even managing to get there.
It's a standard store app that passed WACK, it's live on the store and I've not done anything weird. It's a combination of C# and F#, but the entire UI/XAML stuff is C#.
Completely stumped at this point, not sure what to do.