I got the same model wireless keyboard and the same problem and many be I could explian why the "alu foil" solution works.
Actually what's under the + contact is a some kind of pallet(with a whole in the center), I guess it is for avoid short circuit. dont know how, it's been flip over....(mine is maybe because of the transportation from one city to another), then the + contact can not get through the whole, that's why you insert a "steel foil" could help current to find it's way.
I found a hardware engineer who disassembled the keyboard and fixed it. hope this could help some one, maybe I didn't explain it clear, if you are confused, feel free to send me an email: springrider at applancer.com. I'm happy to help people out of this annoying problem, really annoying....