Explanation of polkitd Unregistered Authentication Agent

Policykit is a system daemon and policykit authentication agent is used to verify identity of the user before executing actions. The messages logged in /var/log/secure show that an authentication agent is registered when user logs in and it gets unregistered when user logs out. These messages are harmless and can be safely ignored.

Different service trying to run on the same port

• I encoutered similar error when running newly installed nginx. Turns out that port 80 is being used by apache. Updating the default port fixed the problem

  local@machine:/$ sudo lsof -i -P -n | grep LISTEN 
  
  local@machine:/$ sudo vim etc/nginx/default
  
  server 
  {
  
   listen 86 default_server;
   listen [::]:86 default_server;
   .....
   ..
  }