@ulrich using your code and 40 parallel publishers, I do not hit your error (tried on a few different systems and let it run for a while). Are there any other details you can provide?
Unrelated to the problem, but you can have multiple direct publishers on the same messaging service (e.g. share the messaging service outside of the goroutines) instead of a separate messaging service per goroutine. This will use fewer client and server side resources.