The error stack is showing a bunch of obfuscated VS functions. I'd say something in your note tags on the skill, or in the action sequence, is trying to get the id of the target, and it doesn't exist. Or perhaps some state it has is running a <before eval> or something with some bad code in it.
If it only happens to this one enemy, the best thing to do is first to remake the troop from scratch, and if it still happens, delete and remake the enemy, and then remake the troop again.