I had the same problem with JBOSS EAP 6, it took me 2 days for find a solution !.
The cause is that your agent need to create a Logger at statup, he need to be able to access the logmanager classes before JBOSS Modules has been initialized. You need to add JBoss LogManager to the boot classloader. Then, there will be a conflict between LogManager available via ModuleClassLoader and classes loaded via system classloader.
The solution is to make Java Agent and JBoss Modules use the same classloader to load the LogManager classes.
For EAP 6, In your standalone.conf (or domain) (It must be close for your version)
add
JAVA_OPTS="$JAVA_OPTS -Djava.util.logging.manager=org.jboss.logmanager.LogManager"
JAVA_OPTS="$JAVA_OPTS -Xbootclasspath/p:$JBOSS_HOME/modules/org/jboss/logmanager/main/jboss-logmanager-1.3.1.jar"
and for make Java Agent and JBoss Modules use the same classloader
modify this piece of code in adding org.jboss.logmanager like this :
if [ "x$JBOSS_MODULES_SYSTEM_PKGS" = "x" ]; then
JBOSS_MODULES_SYSTEM_PKGS="org.jboss.byteman,org.jboss.logmanager"
fi
Hope this will help.