The OG was 1057 but the FG was 1020. Thus, the ABV is actually 4.86% and not 5.88% as automatically generated above.
I used the "sour beer" technique (i.e. collecting ~300 ml of the wort before adding the hops, leave it covered in film in a cold and dark place for 3 days, collecting ~150ml of that soured wort and boiling it for ~10 minutes, cool it down and add it to the fermenting wort).
I tried this beer and tastes GREAT. It's also a very good base for further experiments and additions, such as coffee, vanilla or chocolate.