nevermind, the following ERROR regarding snap store caused the upgrade
problem.
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https://bugs.launchpad.net/bugs/1985253
Title:
In a new 20.04 LXD container, do-release-upgrade crashes
unfortunatly the issue seems not to be fixed in my case. I'm running
Distributor ID: Ubuntu
Description:Ubuntu 20.04.6 LTS
Release:20.04
Codename: focal
in proxmox container.
apt update
Hit:1 http://archive.ubuntu.com/ubuntu focal InRelease
Hit:2
