the following :
was solved by setting permissions for the user at the Proxmox backup server .
Code:
# pvesm add pbs fbc-lt --datastore store3 .....
create storage failed: error during cfs-locked 'file-storage_cfg' operation: proxmox-backup-client failed: Error: HTTP Error 403 Forbidden: permission check failed
# syslog:
Jul 20 19:36:45 s022 proxmox-backup-proxy[10881]: GET /api2/json/admin/datastore/fbc-long-term-backups/status: 403 Forbidden: [client [::ffff:172.30.24.4]:42770] permission check failed
was solved by setting permissions for the user at the Proxmox backup server .