I've found out the problem ...
I have a JMS connection in the session bean that "loose" the authentication ...
.. and it seems that there are bugs in JBoss 5.1 together with JMS 1.4 (look [url]http://community.jboss.org/thread/44409?tstart=0[/url])
Solved by updating to JMS 2.
Thanks to everyone.
D.