Upgraded MYOB from 2019 to 2023 - Can no longer access using C# API based toolset

I had to upgrade my ~2019 MYOB installation to 2023. Now my C# application can no longer read the upgraded database files(s). I have fully installed AccountRight 2023.8.0 which also installed the latest AddOnConnector, which is running. The first code encountered which references the datafile is:

private void CheckInvoiceRecords()
{
try
{
string pageFilter = string.Format("$top={0}&$skip={1}&$orderby=Date desc", _PageSizeInv, _PageSizeInv * (_CurrentPageInv - 1));
var invoiceSvc = new InvoiceService(MyConfiguration, null, null);
invoiceSvc.GetRange(MyCompanyFile, pageFilter, MyCredentials, OnComplete_inv, OnError_inv);
}
catch (Exception ex)
{
_clk = clockMarker.Idle_Goto;
_ErrorList = "Invoices read Stopped: " + ex.Message;
}
}

and this returns a 404 not found error. 
Browsing to http://localhost:8080/AccountRight/ displays a single line table showing the correct product version (2023.8) and the full Uri (http://localhost:8080/AccountRight/xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxx) xxx edited manually of course... 

Folloeing this results in a HTML page showing:
[Message, The Company File version is not supported.][Message, The Company File version is not supported.]

So what isn't supported ? What has changed ?