Wanted to say to Till and Falko: thank you for good work. Updating from 2.2.13 to 2.2.21 has worked fine.
Before running the setup script I've executed this as fallback:
# Backup script for ISPConfig-2.x.x installations
# It should be executed before updating inside the 2.x.x branch
# from one version to a higher one.
# After executing you get a minimum data collection to be able to revert
# the update process by hand in case an update to a higher 2.x.x version
# of ISPConfig fails.
# This script is !! NOT !! backing up any www-data or other configuration
# files, that are necessary to run an ISPConfig Server!!
[ -d $BKPDIR ] || mkdir $BKPDIR
echo '1.) Backing up /home/admispconfig to file home_admispconfig.tar ...'
tar cpf $BKPDIR/home_admispconfig.tar --exclude='*/temp/*' /home/admispconfig && echo ' home_admispconfig.tar ==> OK'
echo '2.) Backing up /root/ispconfig to file root_ispconfig.tar ...'
tar cpf $BKPDIR/root_ispconfig.tar /root/ispconfig && echo ' root_ispconfig.tar ==> OK'
echo '3.) Backing up ISPConfig database 'db_ispconfig' to file db_ispconfig.sql'
echo 'Password for mysql root is required'
mysqldump -p -u root db_ispconfig > $BKPDIR/db_ispconfig.sql && echo ' db_ispconfig.sql ==> OK'
echo 'IF YOU CAN SEE 3 TIMES THE "==> OK" MESSAGE THEN YOU SHOULD BE SAFE.'
echo 'ANYWAY: CHECK NOW THE 3 FILES BY HAND - AND THEN PERFORM YOUR UPDATE.'
echo Backup directory is: $BKPDIR
ls -lh $BKPDIR
Well, this is surely not perfect but executing something like this before
the official setup script makes me feel better while updating.