Ran into an issue where user was not monitoring the drive space available on their SFTP server / DRS backup target and ultimately filled the drive. Unfortunately UCCX doesn't care when attempting scheduled backups how much space is available and ultimately backup will fail.
What's worse is checking the DRS backup status and finding a backup in progress from days or weeks ago. UCCX gives you a pleasant Cancel Backup button to press, but that may result in a infinite Cancelling Backup status loop, refreshing every 5 seconds.
SSH to the UCCX, Primary first if you have HA. then run:
utils service restart Cisco DRF Master
wait for that to complete and then run
utils service restart Cisco DRF Local
If you have a second HA server, repeat the process there.
The good news is this is not business impacting.