If you're on 6.3, check out those new services you have running on the KServer front-end box, like the Plugin service and such-like. I'll bet that one of them has given up the ghost and needs to be kicked.
I've opened a series of tickets about this and have basically been told that "there's a hotfix for that" only to have the hotfix come along and I see the problem again a few weeks later anyway. I've also been told that there's essentially no way to monitor for when these new extra services break except to just... notice that you're not getting emails/alerts/script-processing. Um.