When perfoming an “apt-get upgrade” on your machine, and you have both PowerDNS and Plesk installed, you may encounter a cumbersome situation:

Plesk automatically tries to reactivate bind9. However, bind9 and PowerDNS may be listening on the same UDP port (53). The best countermeasure against that is

sudo service bind9 stop
sudo service bind9 disable

Additionally, Debian and PowerDNS aren’t really friends. The problem mentioned at https://github.com/PowerDNS/pdns/issues/4219 which is resolved with https://github.com/PowerDNS/pdns/issues/4219#issuecomment-234250149 still isn’t handled out of the box. So, additionally, you should comment the line

RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6

in /lib/systemd/system/pdns.service. This adjustment may get overwritten with each upgrade that you perform. Afterwards do a

sudo systemctl daemon-reload
sudo service pdns restart

If you forget to comment the line mentioned above, you may end up with the strange error message:

pdns.service: Failed at step ADDRESS_FAMILIES spawning /usr/sbin/pdns_server: Invalid argument