Final straw??? Simple Store apps throw exceptions on startup. Any idea why?

As my week has been so badly disrupted by software updates breaking things (BOTH Microsoft and Xamarin this week), I thought I'd take a step back and have a go at getting a Windows Store app running, before trying to recover from all the broken bits next week (I've already wasted a day sorting out references, Nuget packages etc).

Unfortunately, everything I try in terms of getting a Store app running ends up in an exception, with the following stack trace. The inner exception seems to be "Error HRESULT E_FAIL has been returned from a call to a COM component".

at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)\r\n
at System.Threading.Tasks.Task

1.GetResultCore(Boolean waitCompletionNotification)\r\n
   at System.Threading.Tasks.Task

1.get_Result()\r\n
at Xamarin.Forms.Platform.WinRT.WindowsBasePlatformServices.GetAssemblies()\r\n
at Xamarin.Forms.Device.GetAssemblies()\r\n
at Xamarin.Forms.Registrar.RegisterAll(Type[] attrTypes)\r\n
at Xamarin.Forms.Forms.Init(LaunchActivatedEventArgs launchActivatedEventArgs)\r\n
at WinUniversal.App.OnLaunched(LaunchActivatedEventArgs e)" string

Whether it's my own solution, or one of the samples, such as the Working with Colors sample ( https://github.com/conceptdev/xamarin-forms-windows-preview ) or the Hanselman app, they all fail in the same way.

Could this be related to certificates/keys? My understanding is that on a local debug build, that shouldn't be an issue - am I wrong? Any help would be much appreciated.

After this week, I am within a hair's breadth of giving up on Xamarin (after 4.5 months) and taking a different approach, where poorly tested updates and poor/incomplete documentation are less likely to waste my time. If I cannot get a simple Store sample to work locally, that could be the final straw. I've taken a look at what is happening with procmon, but nothing is jumping out at me. I've even run as administrator etc just in case, but to no avail.

So:
(1) has anybody got Store apps working?
(2) has anybody got any idea what is causing that exception/stack trace?

Thanks,

John H.