Proxmox 1.9 – not fully updated (1.9 build 6542)
Had backups set to backup snapshots of machines every day starting at 2AM. They generally took about 3 hours and would finish by 5AM.

Little did I realize that I was no longer receiving the emails saying “backup successful”. Last one was 4 weeks ago. Whoops doesn’t even start to cover that.

Checking it out it looks like the backup stalled on one of the virtual machines.
ps auxww |grep vzdump
Then just kill any PID that looks like vzdump is running.

Re-run the command to backup either from the GUI or from the command line:
vzdump --quiet --snapshot --storage STORAGE --maxfiles 5 --mailto admin 101 102 103 etc