Looking via druid overlord console the merge tasks that fail do not contain any logs. If I click on logs it says “No log was found for this task. The task may not exist, or it may not have begun running yet.”
This seems like a bug. If noone knows anything about it then I will just log it as a bug.
I did find http://druid.io/docs/latest/configuration/indexing-service.html#task-logging about logging. But my success task logs are being created without any issues.