Pixhawk communication via USB on Linux


I received my Pixhawk, and tried to load the ArduPlane firmware to it from Mission Planner (on Windows), everything worked fine, I could see data coming from the board.

After that, I started working on Linux with APM Planner, and I could connect to the board, but there was nothing coming from the Pixhawk. Similar to this thread.
I tried with the latest APM planner, but still the same.
Also added the user to the dialout group, that should be fine.

I also tried the example sketches, I could compile and upload the programs (eg. UART_test), but when I opened a serial monitor, or run mavproxy.py --setup --master /dev/serial/by-id/usb-3D_Robotics_PX4_FMU_v2.x_0-if00 as this tutorial says, there was again nothing.

I assume there is a problem with the communication on Linux (not with APM Planner), but I don’t know what I am doing wrong here. Any ideas?