SSO/Troubleshooting
From Authentication Tools for Joomla! (JAuthTools)
HTTP SSO
HTTP SSO is usually reliant on another system to handle the actual user identification procedure, typically the web server. Web servers are usually very good at doing this and are much more efficient at doing so.
Kerberos Troubleshooting
If you're using Active Directory, be aware that the domain will typically be prefixed onto the provided username (e.g. DOMAIN\username). Because of this you will normally need to put your domain down level name with a slash at the end into the username replacement setting of the SSO - HTTP plugin.