I have the same problem. It seems to stem from the $Proxy5 class being possibly unloaded during the JBoss start up. In my experience I have determined that the class is actually loaded, but I am unable to determine when the class is unloaded. During your start up run it as -verbose and pay attention to classes being loaded and unloaded.
I know how to determine whether the $Proxy5 is loaded, but
I don't know where is the $Proxy5, and how to solve this problem.
make sure you don't have anything that would cause conflicts on your classpath before startup too.