Gazebo doesn't start (but RViz does)

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Hello, I'm quite new to ROS. I started out with the following robot: https://github.com/epfl-lasa/kuka-lwr... I got everything running (so far at least). But there is still an issue, namely RViz starts but Gazebo doesn't. The readme of the kuka-lwr-ros states: "If the Gazebo window does not open this is because a flag is set in the sim.launch file." but I really can't find this flag. The sim.launch looks like follows:

<launch>

<include file="$(find lwr_launch)/launch/simulation.launch">
        <arg name="rviz_config"       value="$(find lwr_simple_example)/launch/rviz/rviz_config.rviz"/>
    <arg name="t1_limits"         value="false"/>
</include>

</launch>

I'd really appreciate your help. Thanks in advance!