akka.actor.ActorLogging does not log the stack trace of exception by logback

Akka has separate logging, which is configured in Akka's application.conf. If you want bridge to SLF4J/Logback - use thеsе settings:

akka {
  loggers = ["akka.event.slf4j.Slf4jLogger"]
  loglevel = "DEBUG"
}

See: http://doc.akka.io/docs/akka/2.0/scala/logging.html

As far as I can see here, reason (Throwable) should be the first argument of log.error:

 def error(cause: Throwable, message: String)

That's why you see "WARNING arguments left" - your Throwable argument was just ignored.