Thanks patrikg!
That got me sparked again.
Solved it.
the rotate command was correctly called by rsyslogd but... the files in the rotate script needed a full path description
rotate script in syslog.conf is called with logfile as argument
$outchannel debug_log,/var/log/debuglog.txt, 100000, /var/log/rotate debuglog.txt
rotate script itself creates full path:
tail -n 500 /var/log/${1} > /var/log/${1}.tmp
mv -f /var/log/${1}.tmp /var/log/${1}
and its working. thanks for the spark