I found what my problem was: I didn't allow the Auction product type to be added to the cart (duh!). The button appears now.

It now seems like the database did not update properly. I have a SQL error:
"1054 Unknown column 'bid_old_buynow_price' in 'field list'
in:
[update product_auction_extra set bid_old_buynow_price = **** where products_id = ****]".

This should be easier to solve.