Okay ... So I haven't reinstalled the firmware since my first message. M115 told me I'm using version 3.1.1 as expected. I tried running it standalone but it didn't work.

Then I reconnected the pi and DWC finally connected.

I don't really get what happened here but fortunately it finally works.

The LED does pulse. However I have already reinstalled the firmware following the procedure you linked.

I erased the firmware with the jumper and used bossa to write Duet3Firmware_MB6HC.bin on the Duet. Maybe I wrote the wrong file ?

There is no SD card in the duet and I have connected it with a ribbon cable like the tutorial said.

I have followed the steps listed here https://duet3d.dozuki.com/Wiki/SBC_Setup_for_Duet_3 however the DWC doesn't work it's stuck on "connecting".

I have tried running

sudo journalctl -u duetcontrolserver -e

and the output looks something like this

Nov 18 17:12:11 duet3 DuetControlServer[15625]: Duet Control Server v3.1.1
Nov 18 17:12:11 duet3 DuetControlServer[15625]: Written by Christian Hammacher for Duet3D
Nov 18 17:12:11 duet3 DuetControlServer[15625]: Licensed under the terms of the GNU Public License Version 3
Nov 18 17:12:12 duet3 DuetControlServer[15625]: [info] Settings loaded
Nov 18 17:12:12 duet3 DuetControlServer[15625]: [info] Environment initialized
Nov 18 17:12:12 duet3 DuetControlServer[15625]: [fatal] Could not connect to Duet (Board is not available (no header))
Nov 18 17:12:12 duet3 systemd[1]: duetcontrolserver.service: Succeeded.
Nov 18 17:12:17 duet3 systemd[1]: duetcontrolserver.service: Service RestartSec=5s expired, scheduling restart.
Nov 18 17:12:17 duet3 systemd[1]: duetcontrolserver.service: Scheduled restart job, restart counter is at 992.
Nov 18 17:12:17 duet3 systemd[1]: Stopped Duet Control Server.
Nov 18 17:12:17 duet3 systemd[1]: Started Duet Control Server.
Nov 18 17:12:18 duet3 DuetControlServer[15640]: Duet Control Server v3.1.1
Nov 18 17:12:18 duet3 DuetControlServer[15640]: Written by Christian Hammacher for Duet3D
Nov 18 17:12:18 duet3 DuetControlServer[15640]: Licensed under the terms of the GNU Public License Version 3
Nov 18 17:12:19 duet3 DuetControlServer[15640]: [info] Settings loaded
Nov 18 17:12:19 duet3 DuetControlServer[15640]: [info] Environment initialized
Nov 18 17:12:19 duet3 DuetControlServer[15640]: [fatal] Could not connect to Duet (Board is not available (no header))

I have only found one topic while googling how to fix that but the guy shipped his duet back ..

I have tried erasing the firmware and installing it anew but I still have the same problem.
Any ideas as to why this is happening ?