While I still think that tcp_wrappers are something to left behind at this point, I agree that the issue is somewhere else and has nothing to do with slapd.
I found a way of overcoming this issue by tuning system resouce limits for the slapd process. Read more about this here: http://ral-arturo.org/2017/02/14/about-process-limits.html If the link is down and someone is looking for this in the future: use prlimit(1) and read getrlimit(2). sorry for the nose, best regards