On Fri, 07 Sep 2018 or thereabouts, Adam Armstrong came forth with:

 Hi,
 
 This is a problem firstly with polling (nothing will discover if the poller
 marks it down).
 
 Do :
 
 ./poller.php -h <host> -d
 
 You might see what's wrong around the point where it tries to work out if
 the device is up or not.
 

Hi again Adam,

Thanks for the guidance, however there is nothing in the output which
gives me a pointer to where the problem is.

FYI, this is the software summary it prints out in case this flags
anything to you:

===
#####  Software versions  #####

 o OS                   Linux 4.15.0-33-generic [amd64] (Ubuntu 18.04)
 o Apache               2.4.29
 o PHP                  7.2.7-0ubuntu0.18.04.2 (OPcache: ENABLED)
 o Python               2.7.15rc1
 o MySQL                5.7.23-0ubuntu0.18.04.1 (extension: mysqli 5.0.12-dev)
 o SNMP                 NET-SNMP 5.7.3
 o RRDtool              1.7.0
 o Fping                4.0 (IPv4 and IPv6)
===

It's not PHP7 related is it? I assume not but....

The strange thing is it's only the 2 Ubuntu 18.04 boxes which are
impacted.