Softbricked twice in a row trying to downgrade

Console: N3DSXL 10.3.0-28U

Edit: after 4 softbricks, I managed to downgrade using a clean(er) SD, formatting system memory, and using OoTHax (I didn't have menuhax installed). OoTHax took way more attempts than menuhax but well, it worked.
I didn't use that FTP-3DS trick or whatever, just plain homebrew launcher with nothing on it but sysUpdater.

I am using @julian20 's build of sysUpdater, installing from menuhax.
On my first "successful" attempt it installed...

Softbricked twice in a row trying to downgrade