Finally this makes sense. The Key Combination for Click-to-Move ON set in Game Helpers is Shift+Alt+N. So, with the ISBoxer addon disabled when you press the Interact with Target's Hotkey, it sends Shift+Alt+N (among other things) to WoW (expecting the addon to interpret it) to turn Click to Move ON. Because of the way WoW handles keystrokes with modifiers, if nothing is bound to Shift+Alt+N it gets interpreted as unmodified N.

Suggestion to OP: I would have been helpful if you had also mentioned that neither inviting, follow or assist worked either.