Comment 23 for bug 2039268

Excluded any snap as culprit by removing all snaps before the upgrade:

$ lxc launch ubuntu:lunar lp-2039268
$ lxc exec lp-2039268 bash
root@lp-2039268:~# snap remove lxd && snap remove core22 && snap remove snapd
root@lp-2039268:~# sed -i -e's/lunar/mantic/' /etc/apt/sources.list && apt update && apt install -y systemd
root@lp-2039268:~# ls -l /run/systemd/generator/snap.mount
-rw-r--r-- 1 root root 274 Oct 26 11:29 /run/systemd/generator/snap.mount
root@lp-2039268:~# snap list
No snaps are installed yet. Try 'snap install hello-world'.

Replacing the snapd-generator as mentioned in comment #17 and adding `ls -l /run/systemd/generator/snap.mount` to the ends of the replacement script shows that /run/systemd/generator/snap.mount is not present at that point of time. Something later creates this file.