C4 Picasso 2014 won't start with button

[Vikis]

Hello. Citroen C4 Picasso 2014, 1.6 diesel, semi-auto. I have strange problems with it. It won't start when clicking the start button. If the ignition is off, I press the brake and push the button. I can hear a click, like the starter is trying to turn the engine. Then a lot of faults on the dash start beeping. After it stops beeping, if I keep the brake pressed and try to start, it says on the screen, "Put the lever in N and press brake to start," although I'm pressing the brake and the lever is in N. If, with the ignition on, I short the starter to positive, it will start without a problem.
What I tried: changed a lot of brake switch sensors, checked all the fuses and relays. Changed the ground cable from gearbox to body, as it seemed a little broken. Changed wheel speed sensor, the fault remained the same.
Interesting faults:
BSI: B1418 00 Fault in seatbelt fastening information status for determining presence of driver
Engine ECU:
U1213 inter systems CAN - one of the parameters present in a data stream sent by the ABS/ESP is invalid
U0304 inter systems CAN - Error in length or prohibited value on transmission ECU data stream
Semi auto ECU:
P171A brake signal fault (switch/CAN) - CAN/by wire consistency
U1213 fault in information transmitted by ABS or ESP or Electric parking brake (EPB) ECU - incorrect value received
ABS:
C1391 00 Fault: engine torque not cleared - Not characterised
C003A:11 Rear right Wheel speed sensor - short circuit to ground
Power steering:
U1213 Communication fault between the ABS/ESP ECU and the Electric Power Steering (EPS) ECU

Additional info:
Gearbox ecu shows engine coolant temperature as -29C. Engine ecu shows gear lever position (A/T information) M, but it is on N. ABS ecu shows engine clearence dialog fault and Electro valve supply relay status faulty.
VIN: VF7**************[VIN obfuscated, can be read by forum staff]

Can anybody help me and advise, what and where to search? Thanks

[mickthemaverick]

From reading your post I got the feeling that you are pressing the start button without the ignition on. If that is the case then it won't start!!

[Vikis]

I wish that was the case — would’ve been an easy fix!

But really, if the ignition is off when pressing the button, I can hear the starter trying to start the engine for a second. If the ignition is on and then trying to start, nothing, just press the brake and put the lever on N

[Vikis]

I still have this problem. Could anyone help me, please?