Hi Marko,
Did you ever find a solution to your problem?
I'm having the same problem, but am running CF on redhat linux.
I too think it's something with quartz getting itself tied in knots.
In my http.log I can see the same as you, the task is triggered but it never completes. The strange thing I have found is that sometimes just resaving a task is enough to start it again.
Here's the thread I started http://forums.adobe.com/message/5811970#5811970
Cheers
Simon