I tried to go back with odin with this tutorial from androidforums (guide-how-unbrick-upgrade-downgrade-your-sga-gt-s5830), but it seems that some driver is missing. I installed kies before, so the driver should be installed.
kies reinstall worked, passed flashing, but now it ends in a boot loop.
ok, data and cache wipe worked
my modifications are gone, but I am happy that the phone is booting again