Below are some helpful locations of files, logs etc for troubleshooting backup errors on WHM
WHM backup logs
Change date to the correct date. Should be one log per day or I guess every time a backup runs.
View WHM backup config
View WHM remote destination config(s)
Replace *** with the appropriate name.
May need to modify this file to increase time out limits if you are having issues with time out errors for backups.
This link has some more info https://forums.cpanel.net/threads/what-commands-does-cpanel-run-over-ssh-to-do-rsync-backups.671777/