So, here is the situation from my side: after rebooting the server, I can access to webadmin. I’ve set a password for root and use with success su - to then execute admin commands.
The different logs about php7.3-fpm don’t provide useful information (for me): logs for apt install --fix-missing | logs for systemctl status php7.3-fpm.service | logs for systemctl status php7.3-fpm.service.
So I simply uninstalled php7.3-fpm and the execute the migration command again. I get error messages related to installed app (like
Error: Failed to read info for cyberchef : Invalid version: '10.19.4~ynh1') but the migration process is ongoing. 