RViz doesn't load .dae mesh. Cannot locate it.

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I am trying to load an underwater robot mesh in RViz. Because I could not do this with a .osg file, I converted it to .dae with Blender. Even though I have done that, RViz is still not able to load it (it doesn't even locate it although, of course, the file is there).

The file is called newestexport.dae.

The error is the following:

[ERROR] [1518623879.761036843]: Could not load resource [robot/nessie/newestexport.dae]: Unable to open file "robot/nessie/newestexport.dae". [ERROR] [1518623879.761668482]: Could not load model 'robot/nessie/newestexport.dae' for link 'nessie_base_link': OGRE EXCEPTION(6:FileNotFoundException): Cannot locate resource robot/nessie/newestexport.dae in resource group Autodetect or any other group. in ResourceGroupManager::openResource at /build/ogre-1.9-mqY1wq/ogre-1.9-1.9.0+dfsg1/OgreMain/src/OgreResourceGroupManager.cpp (line 756)

Also, Meshlab and Blender are able to open the mesh without any problem.

The name of the file is correct. So that is discarded.

Any thoughts?

EDIT: I show the .URDF file that calls the mesh:

    <?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="nessie" >

  <xacro:property name="pi" value="3.141592654" />

<link name="base_link">
      <origin rpy="3.141592654 0 3.141592654" xyz="0 0 0"/>
 </link>

<joint name="base_link_to_nessie_base_link" type="fixed">
    <parent link="base_link"/>
    <child link="nessie_base_link"/>
</joint>

  <link name="nessie_base_link">
    <inertial>
      <mass value="95" />
      <inertia ixx="100"  ixy="0"  ixz="0" iyy="100" iyz="0" izz="100" />
    </inertial>
    <visual>
      <origin rpy="-${pi/2} 0 ${pi}" xyz="-0.2 0 0.75"/>
      <geometry>
        <mesh filename="robot/nessie/newestexport.dae"/>
      </geometry>
    </visual>
  </link>


</robot>

Thank you.

UPDATE: If you change the path "robot/nessie/newestexport.dae" for ithe absolute path "/home/daniel/.uwsim/data/robot/nessie/newestexport.dae" it doesn't work, the error still appears. Nor if you put it in the /home/daniel folder.