NBU-MySQL-Backup appears to be already running and backups fail

In the mysql-zrm.log file we get an error message as follows.

Backup appears to be already running. New backup will wait for the 0 seconds. You can increase this value by adding ‘backup-wait-timeout’ in the backup-set configuration file (/opt/nbu-mysql/etc/mysql-zrm/mysql_test/mysql-zrm.conf). This value should not be more than your backup interval time.
mysql_test:backup:ERROR: mysql-zrm appears to be already running for this backupset
mysql_test:backup:ERROR: If you are sure mysql-zrm is not running, please remove the file /opt/nbu-mysql/etc/mysql-zrm/mysql_test/.mysql-zrm.pid and restart mysql-zrm


Remove the hidden .pid file from the backup-set location (/opt/nbu-mysql/etc/mysql-zrm/mysql_test/) and re run the backup