Comment 2 for bug 1996896

The root cause is indeed the same in both three reports. The proxy server returns HTTP 403 Forbidden when trying to download files through apt-get update.

Nov 17 13:57:42 ubuntu-server subiquity_log.2216[3059]: Err:5 http://ports.ubuntu.com/ubuntu-ports kinetic InRelease
Nov 17 13:57:42 ubuntu-server subiquity_log.2216[3059]: 403 Forbidden [IP: 185.125.188.85 3128]
Nov 17 13:57:42 ubuntu-server subiquity_log.2216[3059]: Err:6 http://ports.ubuntu.com/ubuntu-ports kinetic-updates InRelease
Nov 17 13:57:42 ubuntu-server subiquity_log.2216[3059]: 403 Forbidden [IP: 185.125.188.85 3128]
Nov 17 13:57:42 ubuntu-server subiquity_event.2216[2216]: curtin command in-target
Nov 17 13:57:42 ubuntu-server subiquity_log.2216[3059]: Err:7 http://ports.ubuntu.com/ubuntu-ports kinetic-backports InRelease
Nov 17 13:57:42 ubuntu-server subiquity_log.2216[3059]: 403 Forbidden [IP: 185.125.188.85 3128]
Nov 17 13:57:43 ubuntu-server subiquity_log.2216[3059]: Err:8 http://ports.ubuntu.com/ubuntu-ports kinetic-security InRelease
Nov 17 13:57:43 ubuntu-server subiquity_log.2216[3059]: 403 Forbidden [IP: 185.125.188.85 3128]

You can try reaching the website manually through your proxy, and check if additional settings are needed.

 $ curl --proxy http://squid.internal:3128 http://ports.ubuntu.com/ubuntu-ports

On a side note, there is ongoing work in subiquity to make it show the output of `$ apt-get update` to the user ; and to give the user an opportunity to fix any issue by adjusting some of the settings. In other words, we are planning on "testing" the mirror before proceeding with the install. Hopefully this should give the user a better experience.

As for your test, maybe you could try a different proxy :)