I have the same issue. "Failed to mount CTRNAND." Iam only able start Hourglass9 and the luma3ds configuration.
Tryed to restore my SysNAND (NANDmin.bin) in this case i get the messages "not a proper NAND backup for this 3DS" and "SysNAND Restore (keep a91h): failed!". Help, pls! (New 3ds XL)