AAP fails with Linux guest, even if excluded from AAP

[Moebius]

I am using v13 (Windows-based) for the first time.
On Proxmox v9 a backup job backs up some Windows vms and a Linux (Ubuntu) vm. AAP is enabled but the Linux vm is excluded in the AAP options.
The Linux vm *always* fails to back up with the message:
"Failed to perform backup: Failed to prepare disks for backup"
unless it is powered off, or if AAP is disabled for the whole job. The Windows vms back up nicely.

I cloned this vm to an identical one, and the backup jobs fail the same way.
Another identical Linux vm, with the same settings (hardware, options, etc.), backs up with no issues in Veeam v12 (of course without AAP, since it is not available for Proxmox in that version).

Is there a known issue about this that I'm not aware of?