O arquivo do Clearsigned não é válido, tem 'NOSPLIT' (a rede requer autenticação?) no VirtualBox

Question

Acabei de instalar o Ubuntu em uma máquina virtual hospedada pelo Windows 10

Já faz algumas horas que estou tentando encontrar uma solução para esse problema:

Get:1 http://archive.ubuntu.com/ubuntu xenial InRelease [3,548 B]
Err:1 http://archive.ubuntu.com/ubuntu xenial InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Get:2 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [3,556 B]
Err:2 http://archive.ubuntu.com/ubuntu xenial-updates InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Get:3 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [3,558 B]
Err:3 http://archive.ubuntu.com/ubuntu xenial-backports InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Get:4 http://archive.ubuntu.com/ubuntu xenial-security InRelease [3,557 B]
Err:4 http://archive.ubuntu.com/ubuntu xenial-security InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Fetched 14.2 kB in 4s (3,174 B/s)
Reading package lists... Done
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-updates/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-backports/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/xenial-security/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: Some index files failed to download. They have been ignored, or old ones used instead.

• Minha conexão está funcionando bem: (no Firefox)

• Quandousoowget,receboumstatus200:

Euencontreiestaperguntasimular sudo apt update sempre dando Clearsigned arquivo não é válido, tem 'NOSPLIT' (a rede requer autenticação?) mas existem apenas duas respostas:

• Um: sugere que é um problema de rede específico do meu provedor. (Como posso ter certeza? Então, o que posso fazer para resolvê-lo)
• Dois: é um problema de proxy, portanto, faço a seguinte configuração

O que devo substituir <proxy> ?