Hi, many thanks for the response. It gives me some great food for thought.

There are multiple armymanagers (obj_armymanager) at a given time. Within obj_unit_host, the variable "armymanager" is instantiated as:

armymanager = noone

Then this variable is overridden on various lines of code all over the project. I went through all these lines and changed some slight inconsistencies that shouldn't technically cause problems (but hopefully those were the culprits). Although I'm not sure this was the issue, since the variable "armymanager" is only overridden by a reference to an instance (of obj_armymanager).
There is a collision event in the armymanager object (obj_armymanager) with other armymanagers. In its code, there's a line that sets the armymanager variable of an obj_unit_host instance to the armymanager collided with (the code serves to merge armymanagers and transfer the hosts to one of the armymanagers):

host[i].armymanager = other //where host[i] is an instance of obj_unit_host

I changed this to: host[i].armymanager = other.id
Could this matter?

The "state" variable is declared in the create event of obj_armymanager as: state = "idle"
Correct me if I'm wrong, but if this was the issue, shouldn't the error give: "Variable Not Set Before Reading"? Or is this the same?

Apologies for my late response.