Une autre astuce en anglais (désolé je n'ai pas le temps de traduire) qui correspond à tes symptômes :
"This relates to a steptronic gearbox but other autos should be similar.
Remove the top and bottom plastics around the steering column so that you can get access to the ignition switch.
Pull the gear shift gaiter away at the base and lift up (no need to remove it completely).
Pull up and remove the plate covering the gear shift mechanism (the part the gaiter fits into).
On the back of the ignition switch is a cable (mechanical not electrical, like a bicycle brake cable) connected to the switch by a nut. See item 11 on the REALOEM diagram for STEERING-STEERING COLUMN-STEERING LOCK/IGNITION SWITCH.
This cable runs through the dashboard down to the gear shift mechanism. You should be able to see the other end of the cable clamped at the front-right side of the gear shift mechanism. This end of the cable operates a small thin lever (catch), in my case it was black. The catch stops the gear lever moving from PARK when the the ignition is NOT on. When you turn the ignition key, the ignition switch pushes on the cable and lifts the catch allowing you to move the gear lever. When the car is in anything but PARK, the catch is stopped from the returning to the down position. This stops the cable being pushed back into the ignition barrel and stops the key being removed. The catch/cable arrangement can become stiff and get stuck half way, stopping the gear lever from being moved and stopping the key being removed.
If you can't 0&0&0 the catch on the gear shift mechanism, try moving the key backwards and forward. You might be able to see the cable /catch moving slightly at the gearboc end. Don't force the key.
Once you have found the catch, you can give it a helping hand with your fingers. Move it up while the ignition is turned on and the gear lever should be able to move. Push it down while trying to turn the ignition off and you should be able to remove the key. Also try the opposite way around, I'm writing this from memory.
To free off the mechanism, disconnect the cable at the ignition switch and drip some oil or WD40 down it. Then move down to the end at the gear shift mechanism, try and lubricate the cable and more importantly the pivot point for the catch (it was the pivot that was seized in my case). Move the catch up and down with your fingers to ease the oil into the pivot. It took a few minutes until the catch was really loose. Then move back to the end removed from the ignition switch and push on the internal part of the cable (the cable is heavily spring loaded so i used the flat side of a spanner to push on the end). Keep doing this until the cable moves in and springs back freely (it probably moves back and forth about 5mm). You should keep doing this until a push on the cable lifts the catch enough to move the gear lever and releasing the cable lets it move out and lower the catch.
Screw the cable back onto the ignition switch and check you can get it in gear with the ignition on and remove the key when the lever is in PARK."
Le lien vers le sujet complet : http://www.bimmerforums.co.uk/forum/f15 ... ndex2.html
Après je suis à cours de suggestions, mais des personnes plus qualifiées que moi pourront sûrement t'aider ;)