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

 You do not have any devices in the database called "jr01".

I deleted jr01 yesterday thinking that re-adding it might magically fix
whatever the issue is.

jr01 exists, can be founf on the network and has snmp running.

====
# fping jr01
jr01 is alive

# systemctl status snmpd
● snmpd.service - Simple Network Management Protocol (SNMP) Daemon.
   Loaded: loaded (/lib/systemd/system/snmpd.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2018-09-07 17:59:00 NZST; 17h ago
  Process: 1118 ExecStartPre=/bin/mkdir -p /var/run/agentx (code=exited, status=0/SUCCESS)
 Main PID: 1121 (snmpd)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/snmpd.service
           └─1121 /usr/sbin/snmpd -Lsd -Lf /dev/null -u Debian-snmp -g Debian-snmp -I -smux mteTrigger mteTriggerConf -f


# ps aux | grep snmpd
Debian-+  1121  0.0  0.1  65024 11364 ?        Ss   Sep07   0:44 /usr/sbin/snmpd -Lsd -Lf /dev/null -u Debian-snmp -g Debian-snmp -I -smux mteTrigger mteTriggerConf -f

====

Adding the device jr01 from the command line fails:

====
# ./add_device.php jr01 home v2c 161 udp
Observium CE 18.9.9420
Add Device(s)

Try to add jr01:
Trying v2c community *** [0] ...
No reply on given community *** using v2c.
Could not reach jr01 with given SNMP parameters using v2c.
Devices failed: 1.
====

I'm out of idea's but will go back to first principles and walk the
install guide again just in case I have overlooked anything. jr01 has
previously been discovered okay, but maybe I changed something and don't
remember.

I appreciate your help Adam.