The problem is that the external hard-disk suddenly stops being recognized on the Proxmox server. UUID of the corresponding external hard disk from the FSTAB file was deleted and HDD Proxmox was added again without format.
I am having the same issue - Did some updates on a new Debian10 container, rebooted, and same error,,
I have an external USB connected to the proxmox host - I can seee the backups on the device when logged into the proxmox gui (in fact, I did a backup of the Debian CT on this device
Which fstab file - on the container or the host ?? Maybe disconnect the USB ??