This... (recommendation above)
In Luci, go to admin, and then delete and recreate the dropbear entry and it should work again. You can also set a cron job to automatically restart it with:
30 5 * * * /etc/init.d/dropbear restart
I have mine timed to restart after the nightly IPS signatures download.
Shield Pro v1, Chaos Calmer, FW 1.51 SP1, v8.3.2, Bridge Mode