Have you tried with the headlight unplugged? I can't remember exactly BUT I had a very similar problem and I swear I think it was the headlight itself. I remember getting a bunch of parts (pretty much everything) but nothing worked. I have two 5 Plus, one step through and one non (for what it's worth I think the step through handles better) so I swapped the headlight and Viola! everything worked. At least that's how I remember it. I did and still do have a third party headlight as it was much easier to just buy a 1/2 decent headlight and it's still on the bike as a matter of fact.
