There is a version conflict because both Weblogic and JRuby (1.6.0+) include the JODA time library but the Weblogic version is an older one. To solve it is necessary to tell config/weblogic.xml with the following contents

<weblogic-web-app xmlns="http://www.bea.com/ns/weblogic/weblogic-web-app">
  <container-descriptor>
    <prefer-web-inf-classes>true</prefer-web-inf-classes>
  </container-descriptor>
</weblogic-web-app>

To include the file in the war include the following line in the custom configuration file config/warble.rb

config.webinf_files += FileList["config/weblogic.xml"]