As this has happened to few, I'll explain what I do.
Once a week I spray with WD40 the pivot point on the rear brakes.
If you stand over the rear wheel and look straight down at the back of the brakes nearest the car and get someone to apply the handbrake you should see the arm move away from the stop by upto 10mm. If nothing moves then you have the issue. When the handbrake is released the arm should return right to the stop. if not then use a screw drive etc to push the arm back to the stop. might be stiff so spray with WD and keep working until it returns on its own when brake released.
Next check other side and remember to keep lubricated.
Keeping this free also helps with range.
Another check is to feel the wheel temp. It should be cool not warm and definitely not hot after a short journey as it means the pads are rubbing on the disk all the time. I often walk round the Twizy and feel each wheel hub to see if a brake is sticking.