I manage several servers with Webmin/Virtual min, all running the same configuration and setup (Ubuntu 16.04) and on the one running in Europe (strangely only here, not the one hosted in Asia).
I frequently get the error: "too many children (51 > 50)" in the /var/webmin/miniserv.error Always along with many "/usr/bin/perl /usr/share/webmin/miniserv.pl /etc/webmin/miniserv.conf" is ps aux. I didn't count but it could be my 50...
Everytime there is no user or 1 user connected on Webmin and I don't see more from the log (mass access or something)
Once a restart webmin we are back to normal until the next time.
All research I made point to old mail-list without answer or some old post with old version and "solved" in newest version. I use the latest version of Webmin/Virtualmin and all packages are up to date. Also there is no limit of ressource on the server (when it happen there is still plenty of ram/cpu and no heavy workload).