Hi,
currently I'm facing the issue, that I get a black screen if I chainload hekate from SX OS and boot into Atmosphere via fusee-secondary(fss0). But I'm able to boot into Atmosphere if use fusee-primary (payload) from hekate. So:
SX OS -> hekate -> Atmosphere (fss0) [Not Working]
SX OS...
Reformating didn't work for me either, same error - short red display flash and then power off. I have an o3DS, too.
Used your solution from:
https://gbatemp.net/threads/the-status-of-gateway-and-a9lh.427889/page-16#post-6386005