PowerShell EXE/Script Advanced Sensor: Invalid JSON

I'm trying to get a PowerShell script to work with an EXE/Script Advanced Sensor but I can't seem to get PRTG to recognise the JSON output. It says

If I run the script manually on the probe, it outputs good JSON which passes a validation test:

{
    "prtg": {
        "result": [
            {
                "channel": "Departments",
                "value": "0"
            },
            {
                "channel": "Home",
                "value": "0"
            },
            {
                "channel": "Shared",
                "value": "0"
            },
            {
                "channel": "Total",
                "value": "0"
            }
        ]
    }
}

I've enabled "Write EXE result to disk" but the log file simply contains 1,022 null characters which isn't very helpful.

I've seen other posts where the solution has been to use a 'here' string for the Write-Host cmdlet, but that hasn't helped.

This is the script:

# Inspired by these articles:
#   https://kb.paessler.com/en/topic/71662-how-to-monitor-dfs-replication-status
#   http://blog.simonw.se/programmatically-capture-verbose-output-in-a-powershell-variable/

# Requires the following Windows feature to be installed on the PRTG server:
#   Remote Server Administration Tools > Role Administration Tools > File Services Tools > DFS Management Tools

Param
(
    $SourceComputerName = $args[0],
    $DestinationComputerName = $args[1]
)

Import-Module DFSR

$DomainName = (Select-Object -InputObject $([System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain()) -ExpandProperty Name)

Try
{
    $Folders = Get-DfsReplicatedFolder -DomainName $DomainName
}
Catch
{
    Write-Error -Message "Failed to query replication folders"
    Exit 1
}

$Count = @{}
$TotalCount = 0

foreach ($Folder in $Folders)
{
    Try
    {
        $VerboseMessage = $($Null = Get-DfsrBacklog -FolderName $Folder.FolderName -SourceComputerName $SourceComputerName -DestinationComputerName $DestinationComputerName -ErrorAction Stop -Verbose) 4>&1
    }
    Catch
    {
        Write-Error -Message "Failed to query backlog between $SourceComputerName and $DestinationComputerName"
        Exit 1
    }
    if ($VerboseMessage -Like "No backlog for the replicated folder named `"$($Folder.FolderName)`"")
    {
        $BacklogCount = 0
    }
    else
    {
        Try
        {
            $BacklogCount = [int]$($VerboseMessage -replace "The replicated folder has a backlog of files. Replicated folder: `"$($Folder.FolderName)`". Count: (\d+)",'$1')
        }
        Catch
        {
            Write-Warning -Message $_.Exception.Message
            Continue
        }
    }
    $Count.add($($Folder.FolderName), $BacklogCount)
    $TotalCount += $BacklogCount
}

Write-Host @"
{
    "prtg": {
        "result": [
            {
                "channel": "Departments",
                "value": "$($Count.Departments)"
            },
            {
                "channel": "Home",
                "value": "$($Count.Home)"
            },
            {
                "channel": "Shared",
                "value": "$($Count.Shared)"
            },
            {
                "channel": "Total",
                "value": "$TotalCount"
            }
        ]
    }
}
"@

I originally had the Write-Host output being produced within the foreach loop, but after reading about using a 'here' string, I changed it to what you see above, with all the Write-Host output being done at the end and with the channel/folder names hardcoded. Unfortunately it hasn't helped.

I created this test version of the script, which simply mocks up exactly the same output. This works fine with PRTG -- no invalid JSON errors:

$Count = @{
    Departments = 2
    Home = 3
    Shared = 4
}
$TotalCount = 9

Write-Host @"
{
    "prtg": {
        "result": [
            {
                "channel": "Departments",
                "value": "$($Count.Departments)"
            },
            {
                "channel": "Home",
                "value": "$($Count.Home)"
            },
            {
                "channel": "Shared",
                "value": "$($Count.Shared)"
            },
            {
                "channel": "Total",
                "value": "$TotalCount"
            }
        ]
    }
}
"@

Can someone please help me work out why PRTG doesn't like the JSON output from my first script?

Thanks - Simon

exe-script-advanced-sensor json prtg