Did you enable nesting for those LXC? Systemd of newer LXCs like Debian 11 might require this. Starting with PVE7 all LXCs will be created with nesting enabled by default but with PVE6 the default was disabled nesting. So if you created those LXCs with PVE6 nesting is probably not enabled.
Also keep in mind that not all LXCs won't run with PVE7 anymore. Because PVE7 dropped cgroup support so its required that the LXC supports cgroup2. PVE6 suppoted cgroup and cgroup2.
Also keep in mind that not all LXCs won't run with PVE7 anymore. Because PVE7 dropped cgroup support so its required that the LXC supports cgroup2. PVE6 suppoted cgroup and cgroup2.
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