Error: NAK client hostname: access as amandabackup not allowed from amandabackup@backup server hostname: /var/lib/amanda/.amandahosts

This error is due to missing entries in .amandahost of the backup client. Edit the /var/lib/amanda/.amandahosts on the backup client and add a line

 backup_server_hostname amandabackup

Make sure to use FQDN in all .amandahosts files.