This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You forgot to specify the full path to the log file. So the result is that log4j tries to write to Jetty's current working directory, rather than to its log directory. Which it doesn't have the authority to do, but that doesn't matter because it isn't what you want anyway.