https://docs.mongodb.com/manual/reference/ulimit/#ulimit

A low “soft” ulimit can cause can't create new thread, closing connection errors if the number of connections grows too high. For this reason, it is extremely important to set both ulimit values to the recommended values.

The bold text should be:

failed to create service entry worker thread for in 3.4

and

failed to create thread after accepting new connection, closing connection in 3.2