I think this might be correct. I used HomeMenuHax as my entrypoint and got a semi-brick. After a lot of trial and error, I was able to enter recovery mode and update to 10.3. Only problem now is... I have no Homebrew entrypoint. HomeMenuHax theme is gone and replacing the old extdata doesn't work. xD At least I got my system back, though.
My system was:
O3DSXL, 10.1.0-27U, HomeMenuHax 2.1. "Successfully" downgraded, but got black screen on automatic 10 second reboot. Without SD card, got "an error has occurred"; with SD card, got black screen.
If there's a means to getting homebrew working on a fresh 10.3, I'd love to know.
EDIT:
Hmmm... Strange. Any more specific details? Did you have a custom theme?