...
The most common issue seems to be that backup container (mariadb-galera-backup-init) is unable to start the backup and create the folders, then the validation container (mariadb-backup-validate) fails as paths used are not created.
root@xerces-master-policy-03-nfs:~# kubectl logs dev-policy-mariadb-backup-1639580400-dlqpl
cat: /backup/backup.log: No such file or directory
ls: cannot access '/backup/backup-*': No such file or directory
cp: cannot create directory '/bitnami/mariadb/data/backup': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/bin': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/bitnami': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/boot': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/dev': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/docker-entrypoint-initdb.d': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/etc': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/home': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/lib': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/lib64': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/media': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/mnt': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/opt': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/proc': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/root': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/run': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/sbin': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/srv': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/sys': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/tmp': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/usr': Read-only file system
cp: cannot create directory '/bitnami/mariadb/data/var': Read-only file system
Validation Failed!!!