Gazebo urdf model joint link error

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Hi,

I am working in gazebo on a urdf robot model and would like to attach a manipulator. This manipulator consists of a simple link which is attached to the robot (${namespace}/base_link) via a revolute joint. When I launch the basic robot platform without manipulator, everything is working fine. However, as soon as I add the manipulator link and manipulator_joint1, gazebo crashes. It claims to write the error in the logfile

~/.ros/..../gazebo-1.log

However, this logfile does not exist (I was checking in the complete /.ros log directory)!? Do you have any suggestions, how I can debug the gazebo urdf model, written in xacro? If not, do you have any idea, what could be wrong in the implementation? Thanks a lot! This is the minimal example that I am using

<robot name="robot_with_arm" xmlns:xacro="http://ros.org/wiki/xacro">
  <!-- For the manipulator -->
  <xacro:property name="manipulator_link_length" value="2" /> <!-- Link 1 -->
  <xacro:property name="manipulator_link_width"  value="0.02" /> <!-- Link 1 -->
  <xacro:property name="manipulator_link_mass"   value="0.2" /> <!-- Link 1 -->

     ...  <!-- Here the robot is defined with ${namespace}/base_link  -->

  <!-- Link -->
  <link name="${namespace}/manipulator_link">
    <collision>
      <origin xyz="${manipulator_link_length}/2 0.0 0.0" rpy="0 0 0"/>
      <geometry>
    <box size="${manipulator_link_length} {manipulator_link_width} ${manipulator_link_width} "/>
      </geometry>
    </collision>
    <visual>
      <origin xyz="${manipulator_link_length}/2 0.0 0.0" rpy="0 0 0"/>
      <geometry>
    <box size="{manipulator_link_length} ${manipulator_link_width} ${manipulator_link_width} "/>
      </geometry>
      <material name="black"/>
    </visual>

    <inertial>
      <origin xyz="${manipulator_link_length}/2 0.0 0.0" rpy="0 0 0"/>
      <mass value="${manipulator_link_mass}"/>
      <inertia
  ixx="${manipulator_link_mass / 3 * (manipulator_link_width*manipulator_link_width + manipulator_link_width*manipulator_link_width)}" ixy="0.0" ixz="0.0"
  iyy="${manipulator_link_mass / 3  * (manipulator_link_length*manipulator_link_length + manipulator_link_width*manipulator_link_width)}" iyz="0.0"
  izz="${manipulator_link_mass / 3  * (manipulator_link_width*manipulator_link_width + manipulator_link_length*manipulator_link_length)}"/>
    </inertial>
  </link>

  <!-- Joint -->
  <joint name="manipulator_joint1" type="revolute">
    <parent link="${namespace}/base_link"/>
    <child link="${namespace}/manipulator_link"/>
    <origin xyz="0.0 0.0 0.0" rpy="0.0 ${-pi/2} 0.0"/> 
    <axis xyz="0.0 1.0 0.0"/> 
    <limit lower="-3.141526/2" upper="0"/>
    <dynamics damping="0.7"/>
  </joint>
</robot>