When adding a slave zone in PowerDNS Hosting, we will regularly attempt to transfer the zones to our servers, and thus provide backup nameservers for you.
In order for this to work, please allow zone transfers on your master nameserver from the ip 85.17.219.216.
In BIND, this is done with the option:
allow-transfer {85.17.219.216;};
In PowerDNS, this is done by adding the following line to your pdns.conf
disable-axfr=no
allow-axfr-ips=85.17.219.216