can't communicate with rosmaster on other machine

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Hello everyone,

I have two machines in the same local wifi network. On the machine with the roscore I have set

- ROS_MASTER_URI: http://<own_ip>:11311
- ROS_IP: <own_ip>
- ROS_HOSTNAME: <own_ip> (I know I don't need this one, I just tested it. Also the machines hostname is just localhost.localdomain so I don't know if I can use thatvalue)

On the other machine I have set

- ROS_MASTER_URI: http://<ip_machine_with_master>:11311
- ROS_IP: <own_ip>
- ROS_HOSTNAME: <own_ip>

If I start a roscore and then try rostopic list on the other machine I don't get the error "ERROR: unable to connect to Master!" but the terminal just shows empty space after the command. If I run the command on the machine with the roscore I get displayed that /rosout and /rosout_agg are there. Also I played a rosbag from the other machine which should publish a couple of topics but they also don't arrive at the master.

Could it be a problem that the machines are running different versions of ros? The master uses melodic and the other machine noetic. Any ideas what could be going wrong here? Thanks in advance.