Debian Upgrade Stretch To Buster

This article contains all problems that I encountered when upgrading Debian from 9/Stretch to 10/Buster. Since Debian 9 is already out of standard support, this should not affect a lot of people.

authmysqlrc

"authdaemond: marker line not found in /etc/courier/authmysqlrc" in /var/log/mail.err

Solution: Add these lines at the end of the file:

##NAME: MARKER:0
#
# Do not remove this section from this configuration file. This section
# must be present at the end of this file.

Credits: https://sources.debian.org/src/courier-authlib/0.69.0-2/authmysqlrc/ (found by googling "authmysqlrc example")

"authdaemond: authmysql: cannot parse the MYSQL_OPT setting" in /var/log/mail.err

Add

MYSQL_OPT		0

according to above source.

certbot

snap failed

Installation using snap did not work, weird error messages appear:

An error occurred while fetching Certbot snap plugins: your version of snapd is outdated.
Please run "sudo snap install core; sudo snap refresh core" in your terminal and try again.
An unexpected error occurred:
requests.exceptions.HTTPError: 404 Client Error: Not Found for url: http://snapd/v2/connections?snap=certbot&interface=content

Removing the certbot snap using snap remove certbot did work at the second attempt.

pip worked

Instead, the Python package should be installed as instructed in Certbot Instructions (Apache, pip)