An update for those that care...
The bike shop told us that Haibike asked them for a certain number on a sticker so they know which brake it is. (What?) We couldn't find this sticker anywhere on the bike. They couldn't even tell us where to look! The serial number, registration stamp, etc, are all not good enough; they need this other number. Finally I had to take the protector plate off the motor, which was not easy, and hope it was under there. There was a sticker, but it had the different starting numbers than what they told me it would be...
This was in the beginning of December. We're all still waiting for a response from Haibike. Apparently they take their vacations very seriously and shut down for a month at Christmas time.
In the meantime, someone, either at the bike shop, or Haibike, left out a bolt under the back fender of my wife's bike. Either that or it was not tightened sufficiently, but in any case it is gone. (The bolt is invisible to the rider since it goes from inside the fender directly into the frame. I only discovered it was missing when her brake light stopped working and I took the wheel off to investigate.)
Apparently the fender shifted a little when she put groceries on the rack without her noticing. So then the tyre rubbed through the plastic tubes that protect the wiring to the brake light, and then through the wires themselves.
So no brake light at all now...
Apparently the bike shop ordered the parts in December when I informed them of this problem, but Haibike is closed until the middle of January...