Error while spawning my robot in gazebo using gazebo_ros

edit

I have the following launch file:

<launch>
  <!-- roslaunch arguments -->
  <arg name="paused" default="false"/>
  <arg name="debug" default="false"/>
  <arg name="gui" default="true"/>

  <!-- We resume the logic in empty_world.launch, changing only the name of 
    the world to be launched -->
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="worlds/empty.world" />
    <arg name="paused" value="$(arg paused)" />
    <arg name="debug" value="$(arg debug)" />
    <arg name="gui" value="$(arg gui)" />
    <arg name="use_sim_time" value="true" />
    <arg name="headless" value="false" />
  </include>

  <!-- urdf xml robot description loaded on the Parameter Server-->
  <param name="robot_description" command="$(find xacro)/xacro.py '$(find labrob_description)/urdf/labrob.urdf'" />

  <!-- push robot_description to factory and spawn robot in gazebo -->
  <node name="labrob_spawn" pkg="gazebo_ros" type="spawn_model" output="screen" 
    args="-urdf -param robot_description -model labrob" />
</launch>

I get the following errors, which I cannot understand:

ERROR: cannot launch node of type [gazebo_ros/gzserver]: can't locate node [gzserver] in package [gazebo_ros]
ERROR: cannot launch node of type [gazebo_ros/gzclient]: can't locate node [gzclient] in package [gazebo_ros]
ERROR: cannot launch node of type [gazebo_ros/spawn_model]: can't locate node [spawn_model] in package [gazebo_ros]

My issue is that I could easily run this launch file one month ago when I stopped using ros for a bit and now that I tried again I can't.

Has anything changed about gazebo_ros package? Why do I get this error and what should I change to fix this error?

Thanks for answering,

Chris

1st EDIT:

I tried also something simple as:

roslaunch gazebo_ros empty_world.launch

but I got the following errors:

ERROR: cannot launch node of type [gazebo_ros/gzserver]: can't locate node [gzserver] in package [gazebo_ros]
ERROR: cannot launch node of type [gazebo_ros/gzclient]: can't locate node [gzclient] in package [gazebo_ros]

I am kind of lost on what is going wrong on a simple command like this.

The gazebo command works fine. I also checked the directory /opt/ros/kinetic/share/gazebo_ros and I didn't find any worlds file. Should I remove the gazebo_ros package and install it again ?