UWP: Cannot install appx or appxbundle package

I'm ready to betatest my UWP app on my different computers. For that I'm following the article "Packagin Universal Windows apps Windows 10". After struggling a bit with the different sizes of icons needed to scale the graphics properly, it finally generated the files needed to install it. :)

Visual Studio then moves on to run the "Windows App Certification Kit" (step 10 under "Create an app package"). This should be as easy as just clicking "Next" a bunch of times and just wait, but it fails!

I tried to run the certification manually by starting the certification kit directly from the start menu. When I select a package to certificate, it fails. If I use Visual Studio to "debug" the app in Release, it passes (so at least that's good news).

Following the article How to troubleshoot app package signature errors I looked in the Event Viewer, but can't find any other references to the error codes.

The error message "A resource does not have default or neutral value" is really not helpful when it's not specifying the what resource it's talking about. The link is to the article "Troubleshooting packaging, deployment, and query of Windows Store apps" does not help in my situation either as it only gives "The package can't be registered. Check the AppXDeployment-Server event log for more info." for the error code 0x80073CF6, and that's where I was looking. The other error code (0x80073B06) is not mentioned there at all.

I also get the same error when I try to install the package using the PowerShell script. I install the certificate first (under "Trusted People" as I'm told), and then run the script in powershell. I've tried with and withouth administrator rights. And on three different computers.

Powershell gives this error:

I'm not sure where to take it from here. I was hoping to put this in the Windows Store as I need help beta-testing, but so far it's not looking good! :(

What am I missing or doing wrong?

Please check your package.appxmanifest for something missing in Assets....that may be the cause Deploying universal application results in "package could not be registered"