hydro Gazebo on a Jenkins -> FAILURE: Test Fixture Nodes [...] failed to spawn

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I am running a gazebo simulation on a jenkins server under hydro. I have an issue there.

I do not have this problem when I am either:

• running the simulation locally
• using groovy (on the server, too)

This is the relevant output:

spawn_model script started
[INFO] [WallTime: 1405355874.601154] [0.000000] Loading model xml from ros parameter
[INFO] [WallTime: 1405355874.614810] [0.000000] Waiting for service /gazebo/spawn_urdf_model
spawn_model script started
[INFO] [WallTime: 1405355874.602682] [0.000000] Loading model xml from ros parameter
[INFO] [WallTime: 1405355874.626550] [0.000000] Waiting for service /gazebo/spawn_urdf_model
... logging to /root/.ros/log/rostest-jenkins-aub-ch-24428.log
[ROSUNIT] Outputting test results to /tmp/src/wet/build/test_results/my_navigation_test/rostest-launch_my_test_param_test___navigation_arg__2dnav_ipa_eband-navigation_ipa__true-robot__cob3-6-robot_env__ipa-kitchen-scenarioName__scene2-sim__true-.xml
testnavigation_test_jenkins_aub_ch_24428_6280624626160673722 ... FAILURE!
FAILURE: Test Fixture Nodes ['world_joint_state_publisher', 'joint_state_controller_spawner', 'base_controller_spawner', 'head_controller_spawner', 'arm_controller_spawner', 'sdh_controller_spawner', 'torso_controller_spawner', 'tray_controller_spawner'] failed to launch
  File "/usr/lib/python2.7/unittest/case.py", line 327, in run
    testMethod()
  File "/tmp/src/wet/src/ros_comm/tools/rostest/src/rostest/runner.py", line 122, in fn
    self.assert_(not failed, "Test Fixture Nodes %s failed to launch"%failed)
  File "/usr/lib/python2.7/unittest/case.py", line 420, in assertTrue
    raise self.failureException(msg)

I think this problem depends on the /gazebo/spawn_urdf_model service.
Any Ideas, how to fix this?
Thanks for your help!

Update:

I now looked deeper into the problem: I reinstalled all dependencies from apt packages using ros-hydro-desktop-full I found an error in the logfile of world_state_publisher:

/opt/ros/hydro/lib/robot_state_publisher/state_publisher
[ WARN] [1406034138.074698487]: The 'state_publisher' executable is deprecated. Please use 'robot_state_publisher' instead
[ WARN] [1406034138.106214435]: The root link dummy_link has an inertia specified in the URDF, but KDL does not support a root link with an inertia.  As a workaround, you can add an extra dummy link to your URDF.

It turns out that this link is actually missing in the simulated robot model.
Can anybody help me on that?