Robot collision and visuals not loaded/shown properly

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Greetings. I try to load a Universal Robots UR10 via it's URDF file into Gazebo. The robot spawner does allegedly spawn the robot correctly. Nevertheless, nether the robots visuals (.dae) nor its collisions (.stl) are shown. I can verify that the robot is in the world with all its links and joints according to the 'World' tab, also if the inertia is visualized, the inertia is properly displayed (as seen in the images).

RViz does visualize properly.

Due to it's complexity I tried to load just the base of the robot for a minimal example.

<?xml version="1.0"?>

<robot name="test_ur10" xmlns:xacro="http://wiki.ros.org/xacro">

<link name="world">
    <origin xyz="0 0 1" rpy="0 0 0"/>
</link>

<link name="base">
    <visual>
        <origin xyz="0 0 1" rpy="0 0 0"/>
        <geometry>
            <mesh filename="package://models/meshes/ur10/visual/base.dae" scale="0.001 0.001 0.001"/>
        </geometry>
        <color rgba="${255/255} ${108/255} ${10/255} 1.0"/>
    </visual>
    <collision>
        <origin xyz="0 0 1" rpy="0 0 0"/>
        <geometry>
            <box size="1 1 1"/>
        </geometry>
    </collision>
    <inertial>
        <origin xyz="0 0 1" rpy="0 0 0"/>
        <mass value="1"/>
        <inertia ixx="1" ixy="0.0" ixz="0.0"
                 iyy="1" iyz="0.0"
                 izz="1" />
    </inertial>
</link>

<joint name="wold_joint" type="fixed">
    <parent link="world"/>
    <child link="base"/>
</joint>

</robot>

But even here the visual is not displayed, just the collision (in order to confirm a simple collision loads with the visual).

Note: The meshes are originally from Universal Robots and at least the .stl are checked by me in Fusion 360.

Is there a solution to my problem? Am I missing something?