initramfs error after updating Ubuntu; not RAM

I updated Ubuntu, and upon reboot I got this error screen:

Things I've tried so far:

• Boot Repair from Ubuntu Live on USB; no change; booting into various recovery modes doesn't help, either.
• Memtest for about 90 minutes, revealed no errors.
• Replacing RAM entirely; still no difference.

I'm not afraid of the command line, but I don't have an advanced understanding of Ubuntu.

df command:

    Filesystem     1K-blocks    Used Available Use% Mounted on
udev             4038808       0   4038808   0% /dev
tmpfs             812256    1548    810708   1% /run
/dev/sdc1        7970032 1949540   6020492  25% /cdrom
/dev/loop0       1860480 1860480         0 100% /rofs
/cow             4061264  407568   3653696  11% /
tmpfs            4061264   46844   4014420   2% /dev/shm
tmpfs               5120       8      5112   1% /run/lock
tmpfs            4061264       0   4061264   0% /sys/fs/cgroup
tmpfs            4061264       0   4061264   0% /tmp
tmpfs             812252      52    812200   1% /run/user/999
/dev/loop1         93184   93184         0 100% /snap/core/6350
/dev/loop2         35456   35456         0 100% /snap/gtk-common-themes/818
/dev/loop3        144128  144128         0 100% /snap/gnome-3-26-1604/74
/dev/loop4          2304    2304         0 100% /snap/gnome-calculator/260
/dev/loop5         13312   13312         0 100% /snap/gnome-characters/139
/dev/loop6         14976   14976         0 100% /snap/gnome-logs/45
/dev/loop7          3840    3840         0 100% /snap/gnome-system-monitor/57
ubuntu@ubuntu:~$ 

fdisk-l:

Disk /dev/loop0: 1.8 GiB, 1905045504 bytes, 3720792 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop1: 91 MiB, 95408128 bytes, 186344 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop2: 34.6 MiB, 36216832 bytes, 70736 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop3: 140.7 MiB, 147496960 bytes, 288080 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop4: 2.3 MiB, 2355200 bytes, 4600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop5: 13 MiB, 13619200 bytes, 26600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop6: 14.5 MiB, 15208448 bytes, 29704 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop7: 3.7 MiB, 3878912 bytes, 7576 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/sda: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: E69014CA-EC9D-4A42-A6E0-403744998B9B

Device     Start        End    Sectors  Size Type
/dev/sda1   2048       4095       2048    1M BIOS boot
/dev/sda2   4096 7814035455 7814031360  3.7T Linux filesystem




Disk /dev/sdb: 4.6 TiB, 5000981073920 bytes, 1220942645 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 268431360 bytes
Disklabel type: dos
Disk identifier: 0xa04a0c61

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdb1         256 1220942591 1220942336  4.6T  7 HPFS/NTFS/exFAT


Disk /dev/sdc: 7.6 GiB, 8179140608 bytes, 15974884 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00433060

Device     Boot Start      End  Sectors  Size Id Type
/dev/sdc1  *     2048 15974883 15972836  7.6G  c W95 FAT32 (LBA)
ubuntu@ubuntu:~$ 

lsblk:

ubuntu@ubuntu:~$ sudo lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0    7:0    0   1.8G  1 loop /rofs
loop1    7:1    0    91M  1 loop /snap/core/6350
loop2    7:2    0  34.6M  1 loop /snap/gtk-common-themes/818
loop3    7:3    0 140.7M  1 loop /snap/gnome-3-26-1604/74
loop4    7:4    0   2.3M  1 loop /snap/gnome-calculator/260
loop5    7:5    0    13M  1 loop /snap/gnome-characters/139
loop6    7:6    0  14.5M  1 loop /snap/gnome-logs/45
loop7    7:7    0   3.7M  1 loop /snap/gnome-system-monitor/57
sda      8:0    0   3.7T  0 disk 
├─sda1   8:1    0     1M  0 part 
└─sda2   8:2    0   3.7T  0 part 
sdb      8:16   0   4.6T  0 disk 
└─sdb1   8:17   0   4.6T  0 part 
sdc      8:32   1   7.6G  0 disk 
└─sdc1   8:33   1   7.6G  0 part /cdrom
sr0     11:0    1  1024M  0 rom