Deleting Stale Template Caches Failed
When a task actually fails (as opposed to getting stuck), Craft will log an error message explaining why. To fish it up, open craft/storage/runtime/logs/craft.log, and search for:

Encountered an error running task


Each time the task runner encounters an error, its log message will begin with that, followed by the task’s ID, its type, which step it was on (if applicable), and the error message the actual task returned.

If you’re not seeing it, check craft.log.1, craft.log.2, etc.