I was just getting a similar message, except it read that "TFTP is not running."
However, my situation is a little different, since I wasn't upgrading the NetSUS software because I already have NetSUS 4.2.1. I was just running OS updates on the CentOS 7.4 host. Before that, NetBoot Server had been working fine.
I ran the NetSUS 4.2.1 installer, and it seems to have fixed whatever was wrong. I was able to restart the NetBoot Server, and boot a test computer. I need to update my boot image, so I'll have to test against that when AutoCasperNBI is done making it.