HA Crash while Backup and turn PROXMOX Machine OFF

I installed Samba backup and set it to do a full backup at 3am. The next day I noticed that my HA crashed. My HA runs on a VM in PROXMOX (with autoboot). The VM was suddenly powered off. I then started the VM manually and everything worked again. (no backup)

Then I started a backup manually and the system was switched off again? What can that be (HDD full?!) Do you have any ideas?

Thanks in advance