Why firewalld service fails with "ERROR: Command_Failed" message

Issue

• firewalld service fails with WARNING: '/usr/sbin/iptables-restore -n' failed and ERROR: Command_Failed messages
• Logging onto the machine using ssh is impossible (No route to host message is returned)

[root@rhel ~]# systemctl status firewalld

Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Mon 2017-02-27 12:09:17 EST; 1 day 1h ago
     Docs: man:firewalld(1)
  Process: 26934 ExecStart=/usr/sbin/firewalld --nofork --nopid $FIREWALLD_ARGS (code=exited, status=0/SUCCESS)
 Main PID: 26934 (code=exited, status=0/SUCCESS

Feb 27 12:05:04 example.com firewalld[26934]: WARNING: '/usr/sbin/iptables-restore -n' failed:  <====
Feb 27 12:05:04 example.com firewalld[26934]: WARNING: '/usr/sbin/ip6tables-restore -n' failed: <===
Feb 27 12:05:04 example.com firewalld[26934]: ERROR: COMMAND_FAILED  <====