In the meantime I know that exchanging the button unit does cost around € 450,- (including labour).
I also know that I can order just the button at Ali Expres for around € 5,- (including shipping).
At youtube I found a few video's on how to replace the button, but because of the airbag, I should disconnect the 12V minus first. Sounds logical (and safe).
But I don't know what happens to the i3 when I disconnect the 12V minus.
How does it start up again? Just reconnecting the 12V- ? Or does it require some reset after that? Anyone who replaced a battery