Here is one that runs on the server (Nagios plugin): https://github.com/fukawi2/check_urbackup
And there seems to be monitoring software which looks at the Windows event log and UrBackup can be used with that: A way to remote monitor backup status?
Monitoring the windows event log is a good idea anyway because Windows reports e.g. disk i/o errors there. Monitoring the clients’ disk S.M.A.R.T. status/Windows update status/virus scanner status should be something the monitoring solution does as well.