buildbot exception in on wicket-branch-6.x

[buildbot]

The Buildbot has detected a build exception on builder wicket-branch-6.x while 
building . Full details are available at:
    https://ci.apache.org/builders/wicket-branch-6.x/builds/224

Buildbot URL: https://ci.apache.org/

Buildslave for this Build: bb_slave1_ubuntu

Build Reason: forced: by IRC user <marting> on channel ##wicket: None
Build Source Stamp: HEAD
Blamelist: 

BUILD FAILED: exception upload_1

Sincerely,
 -The Buildbot