NetBackup for Microsoft SQL Server Differential backup of AlwaysOn Availability Group secondary replica fails with Status Code 2 or 6

Problem

NetBackup for Microsoft SQL Serer Differential backup of AlwaysOn Availability Group databases failed with Status Code 2 or 6

Error Message

Database <DB01> is hosted on an Availability Group secondary replica, Differential backups are not supported.

Cause

MSSQL Server does not support the following types of backups on a secondary replica:
■ Full backups
If a full backup takes place on a secondary replica, NetBackup converts the full backup to a copy-only backup.
■ Differential backups
Backups of this type result in a failed backup.
■ Copy-only transaction log backups
Protecting SQL Server availability groups 
Backups of this type result in a failed backup.

Solution

If you want to backup from secondary, please run full backup with copy-only, it doesn't support differential backup.

Or please backup it from primary, which can support full, differential and logs backup.