Ubuntu 20.04 Server randomly crashes and displays visual artifacts on AMD Ryzen

SYSTEM SPESIFICATION

CPU : AMD Ryzen 5 5600G 6 Core 12 Threads

MB : GIGABYTE B550M DS3H

RAM : VenomRX 8GB DDR4 2666MHz

SSD : Adata XPG SX6000 PRO 256GB M.2 - NVMe

GPU : NO GPU, Using internal AMD CPU graphics

The operating system:

Distributor ID: Ubuntu
Description:    Ubuntu 20.04.5 LTS
Release:        20.04
Codename:       focal

BIOS version, dmidecode -s bios-version output: "F14"

This is a fairly new computer that I bought as personal server, hosting files, databases, local web development, etc. I had no problem with it until several days ago, it suddenly crashed and displayed weird green visual artifacts like the photo below.

I suspect it to be memory, swap, or BIOS problem. I already refresh the BIOS settings, turn off the "overclock" settings in the Gigabyte BIOS, but still it randomly crashes.

Here's my sudo lshw -C memory output

*-firmware
       description: BIOS
       vendor: American Megatrends International, LLC.
       physical id: 0
       version: F14
       date: 01/04/2022
       size: 64KiB
       capacity: 32MiB
       capabilities: pci upgrade shadowing cdboot bootselect socketedrom edd int13floppynec int13floppytoshiba int13floppy360 int13floppy1200 int13floppy720 int13floppy2880 int5printscreen int14serial int17printer int10video acpi usb biosbootspecification uefi
  *-memory
       description: System Memory
       physical id: b
       slot: System board or motherboard
       size: 16GiB
     *-bank:0
          description: [empty]
          product: Unknown
          vendor: Unknown
          physical id: 0
          serial: Unknown
          slot: DIMM 0
     *-bank:1
          description: DIMM DDR4 Synchronous Unbuffered (Unregistered) 2666 MHz (0.4 ns)
          product: Unknown
          vendor: Nanya Technology
          physical id: 1
          serial: 00000000
          slot: DIMM 1
          size: 8GiB
          width: 64 bits
          clock: 2666MHz (0.4ns)
     *-bank:2
          description: [empty]
          product: Unknown
          vendor: Unknown
          physical id: 2
          serial: Unknown
          slot: DIMM 0
     *-bank:3
          description: DIMM DDR4 Synchronous Unbuffered (Unregistered) 2666 MHz (0.4 ns)
          product: Unknown
          vendor: Nanya Technology
          physical id: 3
          serial: 00000000
          slot: DIMM 1
          size: 8GiB
          width: 64 bits
          clock: 2666MHz (0.4ns)
  *-cache:0
       description: L1 cache
       physical id: d
       slot: L1 - Cache
       size: 384KiB
       capacity: 384KiB
       clock: 1GHz (1.0ns)
       capabilities: pipeline-burst internal write-back unified
       configuration: level=1
  *-cache:1
       description: L2 cache
       physical id: e
       slot: L2 - Cache
       size: 3MiB
       capacity: 3MiB
       clock: 1GHz (1.0ns)
       capabilities: pipeline-burst internal write-back unified
       configuration: level=2
  *-cache:2
       description: L3 cache
       physical id: f
       slot: L3 - Cache
       size: 16MiB
       capacity: 16MiB
       clock: 1GHz (1.0ns)
       capabilities: pipeline-burst internal write-back unified
       configuration: level=3

the free -h output:

              total        used        free      shared  buff/cache   available
Mem:           15Gi       972Mi        13Gi       1.0Mi       885Mi        13Gi
Swap:         4.0Gi          0B       4.0Gi

and sysctl vm.swappiness

vm.swappiness = 60

I will try to do some memtest as soon as possible to verify that the memory is good.

UPDATE 1 Memtest (via terminal) displays no error whatsoever. I'll try next by using memtest86+ (after BIOS logo)