bastien: sorry if I ask basic questions, I just want to run the basic checklist with you, and avoid to overlook anything useful.
1. Ubuntu Studio Controls - System Tweaks. Do you see "Real Time Permissions Enabled" ?
2. uname -a . do you see lowlatency kernel being listed?
3. if you have any power savings software such as tlp, please disable/uninstall, reboot and see if there is any help.
4. check bios updates, bios usb ports settings, if in doubt, can always revert bios settings to factory default and try again.
5. do a full power off of the whole setup, unplug all power supplies/usb.
6. try and switch to different USB ports in the Motherboard. reboot between changes.