rosserial mbed - Unable to sync with device; possible link problem or link software version mismatch such as hydro rosserial_python with groovy Arduino

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Hello everyone! I am trying to use rosserial mbed on a RISC-V platform: GAP8 on a GAPuino development board. The code flashed on the device is the rosserial_mbed hello world example, but when i try to connect with:

roscore

rosrun rosserial_python serial_node.py /dev/ttyUSB1 _baud 57600

i get this error:

[INFO] [1600161124.857713]: ROS Serial Python Node
[INFO] [1600161124.869253]: Connecting to /dev/ttyUSB1 at 57600 baud
[INFO] [1600161126.978947]: Requesting topics...
[ERROR] [1600161141.983163]: Unable to sync with device; possible link problem or link software version mismatch such as hydro rosserial_python with groovy Arduino

and the output of rosout is:

header: 
  seq: 3
  stamp: 
    secs: 1600163942
    nsecs: 675178050
  frame_id: ''
level: 2
name: "/serial_node"
msg: "Requesting topics..."
file: "SerialClient.py"
function: "SerialClient.requestTopics"
line: 406
topics: [/diagnostics, /rosout]
---
header: 
  seq: 4
  stamp: 
    secs: 1600163957
    nsecs: 679801940
  frame_id: ''
level: 8
name: "/serial_node"
msg: "Unable to sync with device; possible link problem or link software version mismatch\
  \ such as hydro rosserial_python with groovy Arduino"
file: "SerialClient.py"
function: "SerialClient.run"
line: 469
topics: [/diagnostics, /rosout]
---

I tried all baudrate settings and inserted wait functions in various part of the example code, but it doesn't seem a timing problem. I'm wondering at this point if it's a client or server side problem.

I am on Ubuntu 18-04 with ROS-melodic, PlatformIO extension on Visual Studio Code. I have also tried with different versions of Ubuntu and ROS (i.e. Kinetic on Xenial, Noetic on Focal) and the related rosserial branches, but still the same connection error everywhere.

if anyone had any suggestions or advice it would be greatly appreciated.

Thanks in advanced!