Since you said “on top of Debian 11”, I think you installed Debian using the normal installer, and likely have an ext4 filesystem instead of btrfs. This means that Snapshots module has no effect on your system (ext4 does not support snapshots). The error displayed in Snapshots was fixed very recently (https://salsa.debian.org/freedombox-team/freedombox/-/issues/2268). After the update, you should see a message that snapshots are not supported.
Is it possible that you have a desktop environment such as Gnome/KDE/XFCE running in the background? I am wondering if some desktop power management is causing the device to shutdown.