Okay, after a lot of debugging, I seem to have figured out the issue: GMLive doesn't seem to support getting variables from stored instance IDs.
In the script that was breaking, one argument was "_instance", a private variable that would hold an instance's ID. So an object might call this script with info_set_value(instanceID,arg1,arg2).
Inside the script, there were a number of lines that would grab local variables from the instance whose ID was stored in _instance. So, for example, it would say:
var _name = _instance.name;
var_ color = _instance.color;
However, GMLive didn't seem to realize "_instance" was an instance ID.
When I switched _instance to simply be the object's built-in "id" variable, it suddenly stopped throwing errors. For example:
var _name = id.name;
var _color = id.color;
Again, the script worked perfectly when GMLive was removed, so there were no issues with the actual "_instance" variable until GMLive was added. I even tried replacing "_instance" with an object_index an it also worked:
var _name = oEnemy.name;
var _color = oEnemy.color;
So it seems GMLive just can't handle stored instance IDs - if I'm correct. The way my project is set up, this is something really important to me, because many if not most of my scripts rely on supplying instance IDs to work as private variables for their functions.