Take a look at this post by Fredrik Haglund: Enable debug logging for EPiServer Scheduler.
Hope this helps
I would recommend to do your on logging with log4net in your scheduled task. You can read about logging in EPiServer here http://world.episerver.com/Documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-60/Logging/
I would recommend this fine gentleman's blog:
Is there a way to retrieve the stack trace of an exception thrown by a ScheduledTask?
The "History" tab on the admin panel only shows this not very informative message:
"12/5/2011 6:57:35 PM FAILED Exception has been thrown by the target of an invocation. [An item with the same key has already been added.]"