Wireless door lock can only lock

[dj92]

Hi,
I'm encountering a quite strange error:
By fob, I can only lock the doors (this works always), unlocking is impossible most of the time.
What I've done up to now:
-re-paired fob and receiver
-disconnected both the battery of the car and the fob for some time
-checked the lithium battery voltage (seems ok, 3V)

The car's battery seems ok to me as the headlights dim only subtle when cranking.

The fob's led lights up for both keys.
I have used the diagnostic mode of the door lock (interior light blinking) and found out that the opening command isn't received at all.

The lock system itself seems innocent to me as I have no problems using the key at the door and the buttons inside.

I initially guessed there would be an interrupted line from the receiver to the body ECU, but iirc the information is transmitted serially.

I'm not sure whether I should replace the receiver or the fob.
The first one can be had for about 40$, while the latter one is almost impossible to get.