I'd recommend you to set 600 permissions for /opt/etc/crontab itself (see man cron), and 700 for the scripts in cron.* dirs (the latter I'd recommend to be on the safe side --- I mean from the point of view of fulfillment of the cron requirements).

finally, check that something like this: /opt/bin/run-parts /opt/etc/cron.1min executes something when started manually.

PS nice /opt/sbin/transmission_watchdog has unclear meaning without setting of the nice level.