Equens, can you open up a ticket using WHM > Support Center > Contact cPanel or via the link in my signature about this issue? If you believe it might be a bug, please use the Bugs link at the top right of the forum or
http://go.cpanel.net/bugreport link.
Also, is there a reason you do have "queue_only_override = false" set on the machine? I do wonder if a queue setting is impacting it. I am not entirely certain the purpose of that setting after reviewing it in
14. Main configuration location, so if you could explain why it's set on the servers precisely anyway.