For more informations see http://jira.jboss.com/jira/browse/JBAS-1380?page=comments#action_12315651

In version 4.0.1sp1 the following bug is fixed "UIL2 server accept thread can be lost due to unexpected error" but not correct implemented. There are a few improvements due to further version. The server didn't crash now with outOfMemory and the client receives now a clear exception. But, while the server is in this unclear state (time from crash to time of closing network sockets), no new connection will be accepted and all clients hang while this time.

Create a test program wich creates uil2 connections to server (create -> close -> create ...). Run this from 2 or 3 machines ...