403 forbidden with socket.io and iisnode - node.js

Question

I have a node.js socket.io server application that I have just moved to IISNode in IIS 8.5 in Windows Server 2012. It is set up to use https.
It all works fine when I browse locally but when I try to access it with a full domain name, I get error 403 (forbidden) when I try to open the web socket connection.
The same problem happens with my real application as happens with the Dante Devine Comedy socket.io sample. Tomasz Janczuk iisnode Dante sample
The standard sample when browsed to with
https://localhost/dante/server-socketio.js
or with
https://interfaces.xxxxxx.co.uk/enable/testing/dante/server-socketio.js
locally on the same server, opens the socket.io web socket connection with
var client = io.connect('https://localhost', { resource: 'dante/socket.io' });
and works correctly producing the following network capture:
Obviously the above does not work when accessed externally (not localhost) so I change the web socket connection to
var client = io.connect('https://interfaces.xxxxxx.co.uk/enable/testing',
{ resource: 'dante/socket.io' });
and this fails with an error 403 as follows:
my web.config is the same as the one supplied with the Dante sample and contains:
<handlers>
<add name="iisnode-socketio" path="server-socketio.js" verb="*" modules="iisnode" />
</handlers>
<rewrite>
<rules>
<rule name="LogFile" patternSyntax="ECMAScript">
<match url="socket.io"/>
<action type="Rewrite" url="server-socketio.js"/>
</rule>
</rules>
</rewrite>
<webSocket enabled="false" />
I'm new to node.js, socket.io and iisnode and a bit of a novice with IIS so this could be a simple iis/web site/https problem and not related to iisnode.
I'd appreciate it if anyone has any suggestions.
John.
Following CuriousGuy's suggestion, I now get the following network capture showing that the switch to web socket protocol has not happened. The actual error shown is
WebSocket connection to 'wss://interfaces.xxxxxx.co.uk/enable/testing/dante/socket.io/1/websocket/ANNcSf7ghQp66nwSS-7P'
failed: Error during WebSocket handshake: Unexpected response code: 200
I tried changing the server io.set from
io.set('resource', '/dante/socket.io');
to
io.set('resource', '/enable/testing/dante/socket.io');
but that was clearly a mistake because I then get an error that io is undefined which seems a little weird.
If I examine the network traffic in fiddler I see:
and the following error message in fiddler when I tap the last network line:
iisnode encountered an error when processing the request.
HRESULT: 0x6d
HTTP status: 500
HTTP reason: Internal Server Error
You are receiving this HTTP 200 response because
system.webServer/iisnode/#devErrorsEnabled
configuration setting is 'true'.
The server stderr log shows:
debug: served static content /socket.io.js
debug: client authorized
info: handshake authorized o7WE52TfMWPxFoJIZFJJ
debug: setting request GET /dante/socket.io/1/websocket/o7WE52TfMWPxFoJIZFJJ
debug: set heartbeat interval for client o7WE52TfMWPxFoJIZFJJ
warn: websocket connection invalid
info: transport end (undefined)
debug: set close timeout for client o7WE52TfMWPxFoJIZFJJ
debug: cleared close timeout for client o7WE52TfMWPxFoJIZFJJ
debug: cleared heartbeat interval for client o7WE52TfMWPxFoJIZFJJ