When the cPanel backup fails with a "partial failure" error, you may find something similar to this entry in the backup logs at /usr/local/cpanel/logs/cpbackup:
The command “/usr/local/cpanel/bin/cpuwatch” failed to report errors to
the status file. at /usr/local/cpanel/Cpanel/Backup/Utility.pm line 219.
The cpuwatch tool is a service that monitors the server load during the backup process on the system. If the server load gets too high, the backups will be paused until the load is reduced, allowing for the server to remain stable. If for some reason the cpuwatch tool isn't able to function properly, errors like the output from above can happen on the system.
When this happens this usually indicates a problem with the /tmp partition on the machine. The /tmp partition may have run out of disk space, or there may be a cron job that clears data from the /tmp partition that happened to run at the same time as the backup process. It's best to not perform server maintenance on the system while backups are running, so avoiding that will keep this error from happening in the future.