Ramon, in the past 2 years I solved several problems by using contact cleaner and enhancer like Deoxit Gold, I use similar stuff from Philips. For example solved on my car aux fan trouble, which did not work, used the contact enhancer on the 2 relays spades and the connector directly at the aux fan, worked perfect, taillight contacts, GM, LKM etc.
Same with a friends E32 750, also some relays needed contact enhancer spray.
BMW even has a tech info:

Model: All BMW Complaint:
Connectors which carry low current are more likely to form deposits which affect the resistance of the circuit through the plug connector. Depending upon the particular circuit these deposits can cause malfunctions and consequently activate warning lamps and check control indicators.

Remedy:

Part replacement can temporarily stop the problem.

If the connector is the problem then without disconnecting the connector measure the resistance of the circuit running through the connector to make sure the connector has a high resistance. A good resistor will have close to 0 ohms resistance.

Visually examine the male and female terminals in the connector housing by removing them. Look at the integrity of the wire crimps, and in some male terminals the integrity of the weld of the male pin to the terminal. Then reinstall. With a zero residue electrical contact cleaner liberally spray the male and female connectors, and allow the air to dry. Apply Stabilant 22A to both male and female terminals so they are saturated, and reconnect the connectors while they are still wet. When the Stabilant 22A dries it will leave a thin coating of polymer film which is conductive between mating surfaces, and is non-conductive between adjacent pins. It can also prevent the formation of more harmful deposits.