Jelly-ffmpeg6 is not available: how to install ?

[TheDreadPirate]

- "acpi=off" was set at the early beginning of the installation of my new OS because on the consol of my server, several "acpi error" messages were displayed. Thanks to this parameter, non more error message ! So I tried to remove this parameter, and let 'GRUB_CMDLINE_LINUX_DEFAULT="" ' empty

This almost certainly is the cause of your problems. ACPI errors can occur when motherboard makers get lazy with their BIOS.
For example, your motherboard's BIOS tells Linux there are two NICs on your motherboard, when in reality there is only one. Linux will try to read the memory area reserved for that second NIC and doesn't find anything. This results in a ACPI error. Why this happens, IDK. Maybe they reuse portions of their BIOS code for multiple boards?

But ACPI is also used to discover your iGPU.

Your old OS is probably still present. Whether or not you'll be able to actually boot into it again, I don't know. If you need to recover files you should shut down the server IMMEDIATELY and use another PC to find a recovery tool, put that on a USB thumb drive, and boot into that recovery tool to try to scan and recover files from your Debian install.