Mine had the same error. I noticed the pump motor was not starting up and error 21 would popup on the screen!

I opened it up by removing 2 screws. Clicked off the front panel and  removed 2 other screws are under the LCD. Desolder the speaker wires. Disconnect 1 rubber tube that goes to the pressure sensor and the big white connector and remove the board. Now you see the white pump motor. Unscrew 4 long black scews and pull it apart. Now you see on top of the motor a white unbalanced round wheel. This is the problem! It got stuck! Since it has slide down too far. You can feel it when you try to move it (Braun disgn failure). How To prevent this.. Remove the small white wheel. Glue it back but dont push it too far down. So it can turn freely. Make sure there is no extra glue anywhere and let it dry. You can apply 3v with an external power supply  to the motor contacts to test if it works. Put it all together and enjoy your blood pressure. Happy repairing guys! ;)