Delete the arm9loaderhax.bin from the root of the sd and delete the "bak" extension from the original arm9loaderhax.bin.bak you had before updating. Try it again but dont use the nightly version,
Also, if you are on V2 a9lh try deleting the "firmware".bin if you have one in the payloads folder, if you have updated from 5.5 to 6.1 but the try the 1st one I mentioned first.