Nagios core service automatically stopping very often

[MurbiesWalto]

We have Nagios core installed to monitor our servers. recently we have upgraded the OS from Ubuntu-14.04 LTS to Ubuntu-16.04 LTS and also we have upgraded Nagios core to latest version – 4.4.3. After this change problem started like Nagios core service getting stopped automatically, So fix this every time I need to reboot system and start the service manually. From /usr/local/nagios/var/Nagios.log seeing below messages I don’t see any errors in this.

[1560226379] wproc: NOTIFY job 2530 from worker Core Worker 2214 is a non-check helper but exited with return code 36
[1560226379] wproc: host=NAGIOS; service=apache2; contact=ajitkumar
[1560226379] wproc: early_timeout=0; exited_ok=1; wait_status=9216; error_code=0;
[1560226379] wproc: stderr line 01: mail: cannot send message: Process exited with a non-zero status
[1560226379] wproc: stderr line 02: mail: Cannot open file /home/nagios/dead.letter: omeglz echat No such file or directory
ubuntu-16.04
nagios

[RajeshEzrado]

Apache2 service causing problem with Nagios appliction, most of the time it fails with below errors. As workaround I need to start the apache2 service to get Nagios work normal.Jul 19 07:35:01 nagios-server apachectl[25530]: AH00558: apache2: Could not reliably determine the server's fully qualified domain name, omegle voojio shagle using 1... message