@Scavy OK merci pour tout Scavy.

Je vais me familiariser avec tout ça ...

merci pour l'accompagnement et bravo de faire tout ça !

I figured out that the hard drive, on which I installed Recalbox (old install before new graphic card), was taking the lead over the fresh recalbox that I was trying to boot from (a usb key). I think the SHARE partition of the hard drive was auto-mounted and used instead of the SHARE partition of the USB key.

It began to work when I disconnected the hard drive..

Thank you to the folks on Discord that helped and motivated me to troubleshoot this.

Hi @Ubergeeksov

Try this link: https://upgrade.recalbox.com/latest/download-wizard/odroidxu4/recalbox-odroidxu4.img.xz

Short update: I managed to boot the GPi Case 2 and Recalbox without any problems. To be honest, I don't know what the reason was that it didn't work. I didn't really do anything different this time, except that I used the dock on the first boot. I don't know if that was really the problem. The fact is, everything works as it should, even without the dock.