in VB.NET unsupported media type on XML-API request with CookComputing.XmlRpcV2 - vb.net

Question

I have the following function in a VB.NET program that was working for a connection with an outsource system XML-API for years with no problems.
Recently, the responsible team for the specific API moved their system to another address (and probably another system) and claim that there is no change and I will just have to make a change in the address that I connect to.
I changed the address and firstly I had some connectivity problems with the new address ("An existing connection was forcibly closed by the remote host") and discovered that I had to compile my code to an earlier .NET Framework version in order to connect because the authentication protocol from their side was changed to TLS v1.2 instead of TLS v1.1 that they were using in the old address.
Now I get an exception “unsupported media type” from CookComputing.XmlRpcV2 when I try to communicate (fetch data) from the new API.
With some googling, I found that the specific exception has to do with content-type header of the request.
The strange thing is that using Postman I can succesfully get a response with both the old and new address using the same content-type (application/xml). The response from the API team was that their system is set to accept and produce the following content types: text/plain, text/html, and application/xml, so I don't think that the problem is with the content-type header.
Does anyone have any ideas on what else should I try ?
Imports CookComputing.XmlRpc
Imports System.Net
Public Shared Function CommunicateWithApi(ByVal pTest As Boolean,
ByVal pCommand As String,
ByVal pInParamsArray As ArrayList,
ByRef pOutParamsArray As ArrayList,
ByRef pErrorMessage As String) As Boolean
Try
Dim _Api = XmlRpcProxyGen.Create(Of IActionAid)()
If pTest Then
_Api.Url = "https://test.blabla.org/servlet/B2BNK"
Else
_Api.Url = "https://live.blabla.org/servlet/B2BNK"
End If
_Api.XmlEncoding = System.Text.Encoding.GetEncoding("ISO-8859-7")
_Api.XmlRpcMethod = pCommand
_Api.UseIndentation = False
_Api.UseIntTag = True
Dim pParams() As Object = pInParamsArray.ToArray
Dim Response() As Object = {}
Response = _Api.FetchData(pParams)
For i As Integer = 0 To Response.Count - 1
pOutParamsArray.Add(Response(i))
Next
Return True
Catch fex As XmlRpcFaultException
MsgBox((String.Format("Fault Response: {0} ", fex.FaultCode)) + fex.FaultString, MsgBoxStyle.Critical)
pErrorMessage = String.Format("{0:00000000000000000000},{1}", fex.FaultCode, fex.FaultString)
Return False
Catch webEx As WebException
MsgBox("WebException: " + webEx.Message, MsgBoxStyle.Critical)
pErrorMessage = String.Format("70000000000000000000,{0}", webEx.Message)
If webEx.Response IsNot Nothing Then
webEx.Response.Close()
End If
Return False
Catch xmlRpcEx As XmlRpcServerException
MsgBox("XmlRpcServerException: " + xmlRpcEx.Message, MsgBoxStyle.Critical)
pErrorMessage = String.Format("80000000000000000000,{0}", xmlRpcEx.Message)
Return False
Catch defEx As Exception
MsgBox("Exception: " + defEx.Message, MsgBoxStyle.Critical)
pErrorMessage = String.Format("90000000000000000000,{0}", defEx.Message)
Return False
End Try
End Function