package management – Apt-get update fails after installing Ubuntu 20.04

I just installed Ubuntu 20.04 on a laptop previously running Ubuntu 19.04. The complete “Erase disk and install Ubuntu” option was used and the installation medium was a USB stick.

The first thing I try to do after installation is of course sudo apt-get update and bam! Errors:

    Err:6 http://security.ubuntu.com/ubuntu focal-security/main i386 Packages
  Hash Sum mismatch
  Hashes of expected file:
   - Filesize:145444 (weak)
   - SHA256:441dab3c8a255fd5fec066a31c406525dab6a5629029e426170033835738ef0f
   - SHA1:70e9e8dec8b9f753d2ddb0d8696b2714fcdbd2b0 (weak)
   - MD5Sum:04ce44602d56d586cc827d4dae28de89 (weak)
  Hashes of received file:
   - SHA256:925ddfd0bf7dc54656ebd729304c8826c779229eab489e39cbda3b83498c305d
   - SHA1:9de08f4ffa54879375ef93899b8179785e795f9a (weak)
   - MD5Sum:3782b8853b62334b19cd0db4e2c4840b (weak)
   - Filesize:145444 (weak)
  Last modification reported: Thu, 29 Oct 2020 19:16:50 +0000
  Release file created at: Sat, 31 Oct 2020 04:53:35 +0000
Fetched 252 kB in 3s (93.9 kB/s)   
Reading package lists... Done
E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/focal-security/main/binary-i386/by-hash/SHA256/441dab3c8a255fd5fec066a31c406525dab6a5629029e426170033835738ef0f  Hash Sum mismatch
   Hashes of expected file:
    - Filesize:145444 (weak)
    - SHA256:441dab3c8a255fd5fec066a31c406525dab6a5629029e426170033835738ef0f
    - SHA1:70e9e8dec8b9f753d2ddb0d8696b2714fcdbd2b0 (weak)
    - MD5Sum:04ce44602d56d586cc827d4dae28de89 (weak)
   Hashes of received file:
    - SHA256:925ddfd0bf7dc54656ebd729304c8826c779229eab489e39cbda3b83498c305d
    - SHA1:9de08f4ffa54879375ef93899b8179785e795f9a (weak)
    - MD5Sum:3782b8853b62334b19cd0db4e2c4840b (weak)
    - Filesize:145444 (weak)
   Last modification reported: Thu, 29 Oct 2020 19:16:50 +0000
   Release file created at: Sat, 31 Oct 2020 04:53:35 +0000
E: Some index files failed to download. They have been ignored, or old ones used instead.

Also, I cannot install gnome-tweak-tools because of unmet dependencies but I’m guessing that’s because the update won’t work. I’ve seen this same problem reported by several other users but solutions suggested there have either not worked for me or aren’t applicable for my case. This is supposed to be a LTS stable release yet the most basic functionality fails.

So, is there a solution to this that would not require me to scratch my brains out?