Box used FTPS for connecting, but it's not working by using this plugin.
https://support.box.com/entries/23597576-Does-Box-support-FTP-connections-

Error log:

11:47:49 Archiving artifacts
11:47:52 [SCP] Connecting to ftp.box.com
11:49:52 ERROR: Failed to upload files
11:49:52 com.jcraft.jsch.JSchException: connection is closed by foreign host
11:49:52 at com.jcraft.jsch.Session.connect(Session.java:236)
11:49:52 at com.jcraft.jsch.Session.connect(Session.java:150)
11:49:52 at be.certipost.hudson.plugin.SCPSite.createSession(SCPSite.java:153)
11:49:52 at be.certipost.hudson.plugin.SCPRepositoryPublisher.perform(SCPRepositoryPublisher.java:160)
11:49:52 at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
11:49:52 at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
11:49:52 at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:776)
11:49:52 at hudson.model.Build$BuildExecution.post2(Build.java:183)
11:49:52 at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:726)
11:49:52 at hudson.model.Run.execute(Run.java:1600)
11:49:52 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
11:49:52 at hudson.model.ResourceController.execute(ResourceController.java:88)
11:49:52 at hudson.model.Executor.run(Executor.java:241)
11:49:52 Build step 'Publish artifacts to SCP Repository' changed build result to UNSTABLE
11:49:52 FTP: Connecting from host [bob-20130513]
11:49:52 FTP: Connecting with configuration [BOX.com] ...
11:50:20 FTP: Disconnecting configuration [BOX.com] ...
11:50:20 ERROR: Exception when publishing, exception message [IOException caught while copying.]
11:50:20 Notifying upstream projects of job completion
11:50:20 Finished: UNSTABLE