empty uwp game project fails nearly in all certification tests

Hello,

I'm trying to create a package for my game, but I seem to be unable to succeed on the certification tests. Eventually I tried to create an empty game project with a single Cube. Unfortunately it fails the same way.

I get messages like: Bytecode generation Error Found: The bytecode generation test detected the following errors: System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.Windows.SoftwareLogo.Tests.Utility.IsPackageWWA(AppXPackage package) at Microsoft.Windows.SoftwareLogo.Tests.BytecodeGenerationCheck.BytecodeGenerationCheck.ExecuteSharedValidation(String manifestFilePath, String packageFullName) at Microsoft.Windows.SoftwareLogo.TestBase.TestBase.ExecuteTest()

or

Background tasks cancelation handler Error Found: The background tasks cancelation handler validation detected the following errors: System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.Windows.SoftwareLogo.Tests.CancelationHandlerValidation.CancelationHandlerValidation.ExecuteSharedValidation(String manifestFilePath, String packageFullName) at Microsoft.Windows.SoftwareLogo.TestBase.TestBase.ExecuteTest()

The common thing is that they are mostly System.NullReferenceException: problems from Microsoft.Windows.SoftwareLogo.TestBase.TestBase.ExecuteTest().

I'm using Unity 2018.2.14f1 and Visual Studio 2017 15.8.9

I've tried all kinds of settings. Now I'm thinking to downgrade to an earlier Unity or Visual Studio. Please give me help!

Thanks, Andris