The key icon problem is caused by a fault in one of the rear door locks.
Diagnosis can occur as shown.

The connection of the rear door locks is shown in the figure below.

Status of DIP switch "ON"
S1 - door closed (external handle locked)
S2 - door open (the switch opens when the door is closed)
S3 - child lock - red sensor (the inner handle is locked after turning the sensor)
S4 - locked (the inner handle is locked when actuated)
The states of the locks are shown in the figure below.

The lock is easiest to check without disassembly in the joint between the body and the door.
Just pull on the cuff. First, unlock the clasp on the top of the cuff. The bottom is hooked behind the protrusion, be careful not to break. After pulling the sleeve back, open the connector lock and pull out the connector.

The fault may be caused by the M2 motor (worn brushes, broken motor windings, or a burnt motor), microswitch S4 (stuck contact) or a stiff operation of the locking mechanism (most often).
In the connector, it is enough to check the condition of the M2 motor by briefly connecting 12V AC to pins 10 and 18. You must hear a distinct click. At the same time, measure the state of the microswitch by connecting an ohmmeter to pins 2 and 13. The value should be 1.97 kOhm closed, 8.2 kOhm open.

The lock is relatively difficult to dismantle. You need to drill 2 dowels, into which holes are then drilled and an M4 thread is cut. Remove the cover, otherwise it is impossible to unscrew the side cover, under which there is access to the motors, microswitches and the locking mechanism.
Sorry, my Polish.