ContextInitializer.autoConfig(LoggerContext) uses current thread context ClassLoader (TCL) nowadays - it calls Thread.currentThread().getContextClassLoader(). Sometimes this classloader has no access to logback classes - for example TCL is parent of classloader, which loaded current class, in oc4j (Oracle J2EE container) at the time of starting servlet container. So logback-*.jar are in WEB-INF/lib directory, but TCL has no access to these jars and to files in WEB-INF/classes.

I think ContextInitializer.autoConfig(LoggerContext) should try this.getClass().getClassLoader() first and only if it fails it should use Thread.currentThread().getContextClassLoader().