[UWP] Unable to activate Windows Store app - The app didn't start

Today, I've been struggling with a Visual Studio 2015 deploy error for a while.

When trying to debug a C# - UWP application, I kept receiving the following error upon application activation:

Places like this have information about other people finding the same error. Some of them even re-installed Windows 10 from scratch with no luck.

In my case, the cause (and solution) was much simpler than that:

I have my all my projects in an external SSD drive. Yesterday I was working in my laptop, and the last compilation I did there was in Release mode. When I brought the external disk back to my main PC today and tried to deploy directly as Debug, the activation failed for some reason.

So, going back to Release mode, rebuild, launch and let Windows activate the app did the trick. After that, I've been able to go back to Debug mode normally. 

Hope it helps.

Cannot find type System.MarshalByRefObject error in UWP app

Playing and learning with UWPs today, I found a strange error when compiling with VS 2015:

Xaml Internal Error error WMC9999: Cannot find type System.MarshalByRefObject

It's a bit misleading, but one can easily see that it's related to DLL references. After dealing with it for a while, I realized some of my Nuget packages were not up to date. Updating them fixed the issue.

More precisely, updating Microsoft.NETCore.UniversalWindowsPlatform to version 5.1.0 fixed the issue.

Hope it helps!