ubuntu – Web Server Failing unexpectedly

We have recently upgraded our server and have been experencing some issues where our website becomes unavailable (returns 500/503 errors)

The server setup is:

Ubuntu 20.04.1
Apache 2.4.41
PHP 7.4.3
OSSEC HIDS

When this issue happens we get a notification from OSSEC with the below:

Rule: 533 fired (level 7) -> "Listened ports status (netstat) changed (new port opened or closed)."
Portion of the log(s):

ossec: output: 'netstat -tan |grep LISTEN |egrep -v '(127.0.0.1| \1)' | sort':
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN     
tcp6       0      0 :::22                   :::*                    LISTEN     
tcp6       0      0 :::25                   :::*                    LISTEN     
tcp6       0      0 :::443                  :::*                    LISTEN     
tcp6       0      0 :::80                   :::*                    LISTEN     
Previous output:
ossec: output: 'netstat -tan |grep LISTEN |egrep -v '(127.0.0.1| \1)' | sort':
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN     
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN     
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN     
tcp6       0      0 :::22                   :::*                    LISTEN     
tcp6       0      0 :::25                   :::*                    LISTEN     
tcp6     512      0 :::443                  :::*                    LISTEN     
tcp6      68      0 :::80                   :::*                    LISTEN     

The only way we have been able to fix this issue is by doing a full server reboot

Any advice on how to diagnose what the issue is would be greatly appreciated, please let me know if there is any information required