Thanks for the links.
I saw these comments before but they didn't help.
I installed JBoss (the same configuration,only some jars with business logic differ) many times in similar environments but I never encountered this problem. Interesting is that shutdown starts 2 hours after the last entry in the server log.
The two lessons from the posts that jaikiran cited are:
1) There is no code within JBossAS itself that causes it to shut itself down automatically.
2) Shutdowns can occur based on a JMX call to shutdown, or on a system signal.
Perhaps your Solaris system has some kind of timer, where after a server has no input for a specific period of time (2 hours), Solaris sends that server a SIGINT.