Couple of things you want to check:
The version of JOSSO supported by Portal today is 1.5. I believe there is a 1.7 out and this is not integrated yet.
Also, back when 1.5 was released, it did not support JBoss 4.2. It had support for JBoss 4.0 only and the actual version I tested on was JBoss 4.0.5.
So with this information in mind, here is what I suggest you try since thats what I have on my end and cannot reproduce your issue:
1/ Use JOSSO 1.5
2/ Use JBoss 4.0.5.
3/ Use JBoss Portal 2.6.3 (this is what I tested). but I think even 2.6.4, and 2.6.5 should work as well since nothing from JOSSO standpoint has changed in those.
I am going to test 2.6.4 and 2.6.5 on my end to confirm, but 2.6.3 works for sure.
Let me know if you still have these issues with JOSSO/Portal.