Gazebo Segmentation Fault with empty world custom launch

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Hello everybody, I am starting to work again on a project on ROS indigo and Gazebo 2.2.3 after a 2 months break and I am experiencing some stressful problems. Everything was working the last time I was using it. Today I tried to launch a complex simulation and gazebo crashed at start showing a segmentation fault error.

To track down the bug I built a very simple simulation consisting in an empty world launch. I realized that the problem is caused by the the models included either in the launch file or in the world file.

If I try to launch the following launch file

<?xml version="1.0"?>
<launch>
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find uav)/worlds/vtol.world"/>
    <arg name="paused" value="false"/>
    <arg name="gui" value="true"/>
  </include>

</launch>

the vtol.world file is the following

<?xml version="1.0"?>
<sdf version="1.4">
  <world name="vtol base world">

    <gui fullscreen='0'>
      <camera name='user_camera'>
        <pose>0.666896 -1.00743 1.30147 0 0.251643 2.20019</pose>
        <view_controller>orbit</view_controller>
      </camera>
    </gui>

    <!-- this also works for ground wheeled robots: do not modify it! -->
    <physics type="ode">
      <gravity>0 0 -9.81</gravity>
      <ode>
        <solver>
          <type>quick</type>
            <min_step_size>0.001</min_step_size>
          <iters>100</iters>
          <sor>1.3</sor>
        </solver>
          <constraints>
          <cfm>0.0001</cfm>
          <erp>0.4</erp>
          <contact_max_correcting_vel>100.0</contact_max_correcting_vel>
        </constraints>
      </ode>
    </physics>

    <include>
      <uri>model://ground_plane</uri>
    </include>
    <include>
      <uri>model://sun</uri>
    </include>
  </world>
</sdf>

Launching this fails and gazebo crashes due to segmentation fault. Removing the following part, solves the issue

    <include>
      <uri>model://ground_plane</uri>
    </include>
    <include>
      <uri>model://sun</uri>
    </include>

where the two referred models are from the gazebo models library. Of course this is not a solutions since I include model quite often. In my .bashrc file I have added the following lines

# gazebo model path
export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:$HOME/Projects/2015a_Nicastro_HeterogeneousMultirobot/devel
export GAZEBO_MODEL_PATH=$GAZEBO_MODEL_PATH:`rospack find uav`/description:`rospack find ugv`/description
export GAZEBO_RESOURCE_PATH=$GAZEBO_RESOURCE_PATH:`rospack find uav`/description:`rospack find uav`/description/materials:`rospack find ugv`/description

I do not understand what is wrong. Has anyone any clue?

Thank you very much.

Andrea