Why is my M.2 SSD is no longer recognised? Replacing the drive is futile; the other SSD is not recognised either

An M.2 SSD (Lexar 970), which has been in use in one of my systems (Mixtile Blade 3 onUbuntu 24.04.4 LTS) for almost one year, is suddenly no longer recognised. It used to appear in lsblk as nvme0n1, but this is no longer the case:

mixtile@blade3n1:~$ lsblk
NAME         MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
loop0          7:0    0 101.4M  1 loop /snap/lxd/39323
loop1          7:1    0  38.7M  1 loop /snap/snapd/23772
loop2          7:2    0  68.9M  1 loop /snap/core22/1912
loop3          7:3    0  61.9M  1 loop /snap/core24/1644
loop4          7:4    0 103.8M  1 loop /snap/lxd/33105
loop5          7:5    0  42.6M  1 loop /snap/snapd/26869
mmcblk0      179:0    0 116.5G  0 disk 
├─mmcblk0p1  179:1    0     4M  0 part 
├─mmcblk0p2  179:2    0   512B  0 part 
└─mmcblk0p3  179:3    0 116.5G  0 part /
mmcblk0boot0 179:32   0     4M  1 disk 
mmcblk0boot1 179:64   0     4M  1 disk 

Neither do fdisk -l and lspci know anything about it. I’ve already surfed through the syslog, but found not even one single entry related to nvme. The drive is simply ignored.

I even swapped the drive for an SSD, which used to be in use in an identical system flawlessly (which has also been reflashed with a new O/S image), and guess what: The other SSD wasn’t recognised either, whereas the original one was recognised by the other system without any problems:

mixtile@blade3n2:~$ lsblk
NAME                                   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
loop0                                    7:0    0 101.4M  1 loop /snap/lxd/39323
loop1                                    7:1    0  68.9M  1 loop /snap/core22/1912
loop2                                    7:2    0  38.7M  1 loop /snap/snapd/23772
loop3                                    7:3    0    69M  1 loop /snap/core22/2412
loop4                                    7:4    0  61.9M  1 loop /snap/core24/1644
loop5                                    7:5    0 103.8M  1 loop /snap/lxd/33105
mmcblk0                                179:0    0 116.5G  0 disk 
├─mmcblk0p1                            179:1    0     4M  0 part 
├─mmcblk0p2                            179:2    0   512B  0 part 
└─mmcblk0p3                            179:3    0 116.5G  0 part /
mmcblk0boot0                           179:32   0     4M  1 disk 
mmcblk0boot1                           179:64   0     4M  1 disk 
nvme0n1                                259:0    0   7.5T  0 disk 
└─ceph--2b07966e--16c5--446c--a8e2--c51edb7e8ee9-osd--block--7d9f708e--66df--4913--9630--2b612299ce24
                                       253:0    0   7.5T  0 lvm  

What’s the problem here? A defective mainboard or chipset maybe?