@lea i literally just had a problem like this, in the end it was a dhcpd error for an app-armor service. The error you ask? Dhcpd isnt installed by default on that os anymore. But they didnt change their configs to reflect that. Anyway journalctl -xeu apt or journalctl -pu err apt might give some insight. Thats how i figuted out it was an apparmor error anyway.