It seems as if you have an other instance of NaviServer (of of some other server like apache?) running already on the same IP address and port.
The important lines of the posted snippet are:
[02/Oct/2019:15:32:10][4508.7f347d230700][-driver:nsssl_v4:0-] Error: nsssl_v4:0: failed to listen on [104.251.214.8]:8443: Address already in use
...
[02/Oct/2019:15:32:10][4508.7f347ca2f700][-driver:nssock_v4:0-] Error: nssock_v4:0: failed to listen on [104.251.214.8]:8000: Address already in use
Normally service files of systemd take care of starting/stopping the server. You can call all running instances on most Unix systems via "sudo killall nsd". For soft shutdowns, NaviServer tries to perform a graceful shutdown, terminating background services, waiting for running jobs etc., so shutdown might take a few seconds.
