USB installer: “ Initramfs unpacking failed: Decoding failed”

It's kind of a bogus error, unless you can't fully boot into Ubuntu.

Either boot into Ubuntu, or into Recovery Mode from the GRUB menu...

• GRUB menu
  • Additional Options
    • Recovery Mode
      • Root Access

...and type the following commands. This will rebuild the (possibly) broken ramdisk file.

sudo update-initramfs -c -k $(uname -r)

reboot