The latest updated changed things quite a bit and did not start. After removing references to syslog-ng.service got it to work with:
/usr/lib/systemd/system/syslog-ng@.service --> /usr/lib/systemd/system/syslog-ng@default.service
# cat /etc/default/syslog-ng@default
#CONFIG_FILE=/etc/syslog-ng.conf
CONFIG_FILE=/etc/syslog-ng/syslog-ng.conf
PERSIST_FILE=/var/lib/syslog-ng/syslog-ng.persist
CONTROL_FILE=/var/lib/syslog-ng/syslog-ng.ctl
PID_FILE=/var/run/syslog-ng.pid
OTHER_OPTIONS="--enable-core"