Error turned out to be file permissions. You need to give the IUSR read rights to the root folder

I've used the script from here: https://our.umbraco.org/wiki/install-and-setup/set-umbraco-folder-permissions-from-command-line

And modified so it set ALL the required permissions and also creates any missing folders.

$PhysicalPath = "C:\inetpub\wwwroot\Umbraco"
$appPoolAccount = "IIS APPPOOL\{application-pool-name}"
$readExecute = $appPoolAccount,"ReadAndExecute","ContainerInherit,ObjectInherit","None","Allow"
$read = $appPoolAccount,"Read","ContainerInherit,ObjectInherit","None","Allow"
$modify = $appPoolAccount,"Modify","ContainerInherit,ObjectInherit","None","Allow"
$fileModify = $appPoolAccount,"Modify","Allow"
$objects = @{}
$objects[""] = $readExecute
$objects["Web.config"] = $fileModify
$objects["App_Code"] = $modify
$objects["App_Data"] = $modify
$objects["bin"] = $modify
$objects["Config"] = $modify
$objects["Css"] = $modify
$objects["MacroScripts"] = $modify
$objects["Masterpages"] = $modify
$objects["Media"] = $modify
$objects["Scripts"] = $modify
$objects["Umbraco"] = $modify
$objects["Umbraco_Client"] = $modify
$objects["UserControls"] = $modify
$objects["Views"] = $modify
$objects["Xslt"] = $modify
foreach($object in $objects.Keys){
    $path = Join-Path $PhysicalPath $object
    New-Item -ItemType Directory -Force -Path $path
    $acl = Get-ACL $path
    $rule = New-Object System.Security.AccessControl.FileSystemAccessRule($objects[$object])
    $acl.AddAccessRule($rule)
    Set-Acl $path $acl
    Get-Acl $path | Format-List
}

$read = "IUSR","Read","ContainerInherit,ObjectInherit","None","Allow"
$acl = Get-ACL $PhysicalPath
$rule = New-Object System.Security.AccessControl.FileSystemAccessRule($read)
$acl.AddAccessRule($rule)
Set-Acl $PhysicalPath $acl
Get-Acl $PhysicalPath | Format-List