OP5 Monitor - How to fix "Cannot push to poller %s, because %s does" when running "mon oconf push" command?

When attempting a mon oconf push you may see this error.

Known possible causes

• You have added a host group to a poller in merlin.conf, perhaps through using mon node add, that does not actually exist. Create the host group first, then add it to Merlin.

Full error examples

mon oconf push

Cannot push to poller %s, because %s does
Traceback (most recent call last):
File "/usr/bin/mon", line 332, in 
ret = commands[cmd](args)
File "/usr/lib64/merlin/mon/oconf.py", line 261, in cmd_push
'not exist' % (name, oconf_file)
TypeError: not all arguments converted during string formatting

mon node status

Traceback (most recent call last):
File "/usr/bin/mon", line 332, in 
ret = commands[cmd](args)
File "/usr/lib64/merlin/mon/node.py", line 69, in cmd_status
sinfo = list(get_merlin_nodeinfo(query_socket))
File "/usr/lib64/merlin/mon/modules/merlin_qh.py", line 12, in get_merlin_nodeinfo
for info in qh.get('#merlin nodeinfo\0'):
File "/usr/lib64/merlin/mon/modules/nagios_qh.py", line 40, in format
yield dict(x.split(kvsep, 1) for x in row.split(pairsep))
ValueError: dictionary update sequence element #0 has length 1; 2 is required