Backups failing with errors similar to one of the following:
Can't run: Data stores 15 transactions limit exceeded (2 / 2)
Can't run: Hypervisor 2 transactions limit exceeded (1 / 1)
Can't run: Backup Server 1 transactions limit exceeded (3 / 3)
Increase the number of simultaneous backups allowed per appliance.
This can be done through the UI: Settings >> Configuration >> Backups / Templates
There is a defined maximum amount of backups permitted to run simultaneously, according to the set configuration.