Today, mechanic Konrad removed the battery cables from my car at his workshop, cleaned them with a wire brush and sandpaper, lubricated the contacts with petroleum jelly, and then reconnected the cables to the battery.
The photos show the before and after cleaning.





I was a bit worried about whether the car would start without any problems, but it turned out the engine started without any problems.
However, after driving a short distance, the BAS/ESP light on the dashboard came on...
I read in the manual that I should perform the following calibration:



When I was ready to perform this procedure, the BAS/ESP light went out on its own and has since stopped flashing.
Maybe this will be useful to someone.

 

Most times the power is disconnected the BAS ESP must be reset. Sometimes driving resets it.

 

This is probably what happened in my car.

 

Some helpful advice:
When disconnecting the battery, always disconnect the negative cable first. Then the positive cable.
When re-connecting the battery, always connect the positive cable first. Then the negative cable.
This will help to prevent any voltage spikes which may lead to electronic equipment troubles.

 

@M60A3Driver Thank you.

 

You're welcome.
You have a good mechanic, so I'm sure that he knew this already. But it's good to be reminded once in a while.