Backup hangs due to over disk quota domain

We have a daily backup schedule to backup all our clients domains. One of the domains had reached its disk quota.

The schedule backup then hangs on the that domain and does not continue backing-up the other domains.

Subsequent scheduled backups the failed due to the hanging backup process.

The hung process had to be manually killed to allow the subsequent backups to run.

After reducing the domain's disk usage, backups successfully again.