Ok so apparently, I was very wrong and I am having my own difficulties. If you have any ideas about how to fix it, they'd be appreciated.
My Ambassador console and retail Cubic worked fine
What FW?
Where did you hear Nintendo slightly altered the NAND on Ambassador 3DSes? I've never heard that and I don't think they did anything of the sort.
As for your issue, make sure you are picking OLD and then all the right numbers. I had no issues on any of my 6 3DSes including the NEW 3DS, after I made sure I was infact selecting the correct NEW/OLD and versions. Also 3 of them had ambassador status so nothing different there.
Where did you hear Nintendo slightly altered the NAND on Ambassador 3DSes? I've never heard that and I don't think they did anything of the sort.
As for your issue, make sure you are picking OLD and then all the right numbers. I had no issues on any of my 6 3DSes including the NEW 3DS, after I made sure I was infact selecting the correct NEW/OLD and versions. Also 3 of them had ambassador status so nothing different there.
Not with a .cia USA. Retail, e-Shop (Japanese), and GW. For the others though.
I'm going to go out on a limb and from what I have read in various threads and even here is the OP is having an issue that is on his end.
The issue OP describes according to the FAQ is he is picking the wrong type (new / old) or firmware
1. Make sure you are using the U ROM.
2. Make sure your 3DS is able to connect to the internet (it downloads an encrypted payload to exploit the sysmodule).
3. Make sure you remove your Cubic Ninja save by going to the menu and holding x+y+l+r.
Do you have US or EUR 3DS ?
For EUR 3DS you might want to try with the EUR CIA of cubic ninja. Using Gateway on emunand with this CIA does work fine for me. If you are using emunand, be sure you select the version of your emunand on ninjhax website.