The group or resource is not in the correct state to perform the requested operation.

private async Task<bool> SetLaunchOnLogin_UWP_Async(bool shouldLaunchOnLogin)
{
    try
    {
        var startupTask = await StartupTask.GetAsync("MyProjectStartupId");

        switch (startupTask.State)
        {
            case StartupTaskState.Disabled:

                System.Diagnostics.Debug.WriteLine("Startup is disabled. Will ask");
                // The code reaches here, but always throws an exception when calling RequestEnableAsync():
                return (await startupTask.RequestEnableAsync()) == StartupTaskState.Enabled;

            case StartupTaskState.Enabled:

                System.Diagnostics.Debug.WriteLine("Startup is enabled. Will disable");
                startupTask.Disable();
                return false;

            default:
                return false;
        }
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine("SpecificPlatformFunctions_UWP - SetLaunchOnLogin_UWP_Async ERROR: " + ex.ToString());
    }

    return false;
}

Startup is disabled. Will ask
Exception thrown: 'System.Exception' in System.Private.CoreLib.ni.dll
SpecificPlatformFunctions_UWP - SetLaunchOnLogin_UWP_Async ERROR: System.Exception: The group or resource is not in the correct state to perform the requested operation. (Exception from HRESULT: 0x8007139F)
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at MyProject.UWP.DependencyServices.SpecificPlatformFunctions_UWP.<SetLaunchOnLogin_UWP_Async>d__13.MoveNext()