config file ... something in your configuration status that you used to reference in one or your registers or vice versa or something about your configuration was pointing to your register and it is not there anymore returning a false status. maybe? I've installed & reinstalled mach 4 so much that i just copy my license and profile -> uninstall -> reinstall -> and configure within 10minutes. That's the efficiency I get for playing around in the screen start up and PLC scripts. haha! Honestly, I have no clue what that error is. Just a guess on my part about "pointing" to a false value or a value that is returning a false or nil status.
could be some type of index that is trying to point to a "key" that had a value of a field in your configuration or startup file which used to be in your register plug-in which you deleted....did you have a table referenced in your register file? your guess is as good as mine...Lua is all about index's that are also known as keys are assigned values in a table or array.
for example: the key "distance mode" = "incremental" and "control mode" = "mill"
index=1 key = distance mode value = incremental
index=2 key = control mode value = mill
if you had a table and deleted it but some code is still trying to pull index#2 of a mystery table that does not exist there would be no key to reference and return an nil or '0' value.
fml, i just totally went down the rabbit hole with this.