Powershell Invoke-WebRequest causing server error - powershell

Question

I have an xml file and I'm using a Powershell script to create a soap request to post it to the url.
The problem I seem to have is with this line of code below producing a server error:
[xml]$res = Invoke-WebRequest $uri -Method POST -ContentType "text/soap+xml;charset=windows-1252" -Body $soapReq
The odd thing is when I open my data XML file in notepad and save it (make no changes), the Powershell script seems to work and transfer the file across to the target url.
I'm writing my xml file in UTF8 charset.
I enclose a tweaked version of Powershell script I am using.
#my db generates this file
$filePath = "C:\MyPath\data.xml"
$result = Get-Content $filePath
#rewrite the file to a specific location and encode it to utf-8
Out-File -FilePath $filePath -InputObject $result -Encoding 'UTF8'
$un = 'un'
$Pw = 'pw'
$xmlData = Get-Content $filePath -Raw
$xmlData = [Security.SecurityElement]::Escape($xmlData)
$soapReq = #"
<?xml version="1.0" encoding="Windows-1252"?>
<env:Envelope xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:env="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ins0="https://url">
<env:Body>
<ins0:Transfer>
<sourceIdentifier>123456</sourceIdentifier>
<userName>$un</userName>
<password>$pw</password>
<xmlDataset><?xml version="1.0" encoding="UTF-8"?>
$xmlData</xmlDataset>
</ins0:Transfer>
</env:Body>
</env:Envelope>
"#
$uri = "https://url"
[xml]$res = Invoke-WebRequest $uri -Method POST -ContentType "text/soap+xml;charset=windows-1252" -Body $soapReq
Before I investigate using vbs to open the file in notepad and save it (overkill), I’m hoping someone can give me advice on how to get this Powershell script working properly.