Old lxd/incus Vms wont boot!

Incus
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2

Welcome!

Can you check whether you are affected by this, Incus 6.5 -- Incus VM not starting in Cluster
Although the title says that’s related to cluster environments, the issue was present also on standalone installations.

  1. Run incus version to verify whether you are really running version 6.5 (affected version).
  2. When Incus 6.5 was first released, there was this bug. It was identified quickly, fixed and new packages were created for Debian and Ubuntu. Perhaps for Arch you need to get refreshed packages? Is there a way to check whether Arch has 6.5 packages that have been updated with the fix?

You mention though that it is an issue with old VMs only, while newer VMs are OK.
When you upgrade to newer versions of Incus, the Qemu virtualization and emulator software may also get updated. Can you verify your version?

$ /opt/incus/bin/qemu-system-x86_64 --version
QEMU emulator version 9.0.2 (v9.0.2-dirty)
Copyright (c) 2003-2024 Fabrice Bellard and the QEMU Project developers
$

There is a chance that there is some kind regression of Qemu 9.0.2 with other images.

On your point, you mention that /boot is empty in those VMs. Can you tell us which images you are running, and also what are the contents of /etc/fstab in the affected VMs. Obviously, we are looking whether those images require the mounting of /boot which somehow did not happen.