NetBackup versions & O/S's for master/media/client.
Type of job being run? EC=6 *could* be just a generic d/b error code, but without any context we could not provide further assistance.
Hence - any log info or job details that you could share that may expound the issue?
See this for some idea of what could be helpful:
it's actually 'stickied' at the top of the forum.