Problem with EXE/XML Sensor

Hi, I have a problem with some Custom EXE/XML sensor. I need to measure load time and bytes received for some web page, but the trick is, that every day I need to use other URL - so I can't just use typical HTTP Advanced Sensor and I wrote some custom sensor in Powershell that should return data I need. The code is as follow:

First I build the proper URL:

[string]$IP = "x.x.x.x"
[string]$Data = [string]((Get-Date).AddDays(-1)).Year+"-"+[string]((Get-Date).AddDays(-1)).Month+"-"+[string]((Get-Date).AddDays(-1)).Day

$URL = "http://$IP/...........&UpdateDate=$Data"

Then I run commands to get page load time, status and data size:

$ResponseTime = Measure-Command {$WebParam = Invoke-WebRequest -Uri $URL -DisableKeepAlive | Select StatusCode, RawContentLength}

$ResponseTimeMS = [math]::Round($ResponseTime.TotalMilliseconds)
$BytesReceived = $WebParam.RawContentLength
$StatusCode = $WebParam.StatusCode

And finally I build output for PRTG:

Write-Host 
"<prtg>"
    "<result>"
        "<channel>Response Time</channel>"
            "<value>$ResponseTimeMS</value>"
            "<Unit>miliseconds</Unit>"
            "<LimitMode>0</LimitMode>"
            "<LimitMaxError>0</LimitMaxError>"
            "<ShowChart>1</ShowChart>"
            "<ShowTable>1</ShowTable>"
    "</result>"
    "<result>"
        "<channel>Bytes Received</channel>"
            "<value>$BytesReceived</value>"
            "<Unit>bytes</Unit>"
            "<LimitMode>0</LimitMode>"
            "<LimitMaxError>0</LimitMaxError>"
            "<ShowChart>1</ShowChart>"
            "<ShowTable>1</ShowTable>"
    "</result>"
    "<result>"
        "<channel>Status Code</channel>"
            "<value>$StatusCode</value>"
            "<Unit>status</Unit>"
            "<LimitMode>0</LimitMode>"
            "<LimitMaxError>0</LimitMaxError>"
            "<ShowChart>1</ShowChart>"
            "<ShowTable>1</ShowTable>"
    "</result>"
"</prtg>"

Everything works perfect when I run sensor directly from the server - it shows proper values, etc. But when I run it from the sensor, it shows strange things - for example StatusCode and BytesReceived are always 0, ResponseTime is definitiv wrong as well. It looks like it actually doesn't ask proper URL, but I have no idea what can be wrong - it's simple code, without remoting, variables in awkward format, etc. Could you take a look and help me with it? Custom EXE sensors are very powerful but because of such problems are sometimes very unintuitive :(

Thanks Marcin