Unable to start snmptrapd.service after some configuration changes

Issue

• The snmptrapd.service cannot be started after some configuration changes.
• The snmptrapd process seems to be running after "systemctl start snmptrapd.service" command but it has been terminted soon.

# systemctl start snmptrapd
Job for snmptrapd.service failed because a timeout was exceeded. See "systemctl status snmptrapd.service" and "journalctl -xe" for details.
# systemctl status snmptrapd.service
* snmptrapd.service - Simple Network Management Protocol (SNMP) Trap Daemon.
   Loaded: loaded (/usr/lib/systemd/system/snmptrapd.service; disabled; vendor preset: disabled)
   Active: failed (Result: timeout) since Tue 2019-03-19 18:35:51 JST; 10s ago
  Process: 27168 ExecStart=/usr/sbin/snmptrapd $OPTIONS -f (code=exited, status=0/SUCCESS)

Mar 19 18:34:21 rhel7snmp.example.com systemd[1]: Starting Simple Network Management Protocol (SNMP) Trap Daemon....
Mar 19 18:34:21 rhel7snmp.example.com snmptrapd[27168]: NET-SNMP version 5.7.2
Mar 19 18:35:51 rhel7snmp.example.com systemd[1]: snmptrapd.service start operation timed out. Terminating.
Mar 19 18:35:51 rhel7snmp.example.com systemd[1]: Failed to start Simple Network Management Protocol (SNMP) Trap Daemon..
Mar 19 18:35:51 rhel7snmp.example.com systemd[1]: Unit snmptrapd.service entered failed state.
Mar 19 18:35:51 rhel7snmp.example.com systemd[1]: snmptrapd.service failed.
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