Hi there, just to clarify have a W221 facelift or a W222 ( your avatar says 2012 so W221)?
On the W221 ( and most likely the W222) you have some actuators that control the release / actuation of the parking shoes when you press the Parking brake button , sothe parking brake module pushes / relaeases the cables of the parking shoes actuators ( which are inside the rear disks). These little actuators tend to get seized with brake shoe dust as years go by, causing issues like this. Please get it checked to prevent damage to the e-Brake.