Debian 6 has been out of support for a long time. Indeed it is high time to update. Use upgrade instructions from Debian Release Notes: https://www.debian.org/releases/wheezy/releasenotes Or did you mean this issue: https://git.ispconfig.org/ispconfig/ispconfig3/-/issues/4613
What I was refering to... Was the indicated How-To is way out of date. It doesn't work for buster. We now generally use MariaDB as well. For those without the need of ISPConfig, this how-to is nearly, but not it at the moment.
https://www.howtoforge.com/installing-powerdns-with-mysql-backend-and-poweradmin-on-debian-squeeze thats the most recent for debian....