MIA,
One thing to verify is that the domain specified in your SSO doc is contained in the Fully qualified Internet host name of the domino server on the Basic tab, and in the Host name field under Internet Protocols > HTTP and that the server's host name is the same as the Fully qualifed host name of the domino server.
Example: Domain specified in SSO doc is: .mydomain.com
The Fully qualified host name field would be something like: myserver.mydomain.com
Also, create a bookmark in your browser with the following in the bookmark properties:
javascript:alert(document.cookie)
Use that bookmark to tell you if you have an LTPAToken after logging in. If you do, then WEBSSO is probably loading on HTTP startup.
Hope this helps.
AJ