The log(String callerFQCN, Priority level, Object message, Throwable t) method of org.apache.log4j.Logger
in the log4j.jar distributed with JBoss throws a NullPointerException if called with t = null. This then
causes an InvocationTargetException on line 288 of org/apache/commons/logging/impl/Log4jProxy.java,
leading to an UndeclaredThrowableException on line 296. The problem is that very often the method is
called with t = null, e.g. on line 235, in the debug method. This then causes some servlets not to work
(Google finds a few instances of this).

The cause is a rather outdated log4j.jar. Replacing log4j.jar by a more recent version seems to
fix this problem