"element not found" = 0x8000490

I'm learning XAML and universal windows apps at the moment. I've encountered an issue where I get an element not found (exception from HRESULT:0x80070490) and I can't seem to figure out where exactly that error is referring to. If I double click on it, VS opens the MainPage.xaml where <Page is underlined with the error.

It's nothing special... Just a collection of a few controls to play with.

<Page
x:Class="App1.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App1"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">

<Grid>
    <Grid.Background>
        <ImageBrush Stretch="Fill" ImageSource="Assets/320x480-1.jpg"/>
    </Grid.Background>

    <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center" Margin="77,23,68,24">
        <TextBox x:Name="HelloMessage" Text="Hello, World!" Margin="10" IsReadOnly="True"/>
        <Button x:Name="ClickMe2" Content="Click Me!"  Margin="10" HorizontalAlignment="Center" Click="ClickMe2_Click"/>
        <Slider x:Name="slider" Margin="10" HorizontalAlignment="Center" VerticalAlignment="Top" Width="100" BorderThickness="0,50,0,0"/>
    </StackPanel> 
</Grid>

This is the exact error as copied from VS.

Severity    Code    Description Project File    Line    Suppression State
Error       Element not found. (Exception from HRESULT: 0x80070490) Control D:\Dev\TestUW\App1\App1\MainPage.xaml   1   

I tried removing the grid and the error persisted. I also verified that App1.MainPage existed as well. How can I figure out what exactly it's complaining about?

Too late now, but I ran into the same problem, copying the code from the MS OpenCV sample link (https://developer.microsoft.com/en-us/windows/iot/samples/opencv)

The problem was that you needed a space " " before the end of the first "page" tag. So instead of the code reading:

mc:Ignorable="d"> <-- missing space after "d"

it should read:

mc:Ignorable="d" >