Finally succeeded in enabling STTRACE and redirecting the stdout when starting syncthing from the synology package manager
After changing the line
start-stop-daemon -b -o -c ${USER} -S -u ${USER} -k 002 -x env HOME=${CONFIG_DIR} ${SYNCTHING} -- ${SYNCTHING_OPTIONS}
to
start-stop-daemon -b -o -c ${USER} -S -u ${USER} -k 002 -x /bin/bash -- -c "env HOME=${CONFIG_DIR} env STTRACE=all ${SYNCTHING} ${SYNCTHING_OPTIONS} > /tmp/syncthing.dbg.log 2>&1"
in /var/packages/syncthing/scripts/start-stop-status
i was able to start synchting through the synology package manager. This can be done from the commandline using de synopkgctl command; /usr/syno/sbin/synopkgctl start syncthing
I now can debug my issue which i reported in thread Continous disk activity