I recently ordered a Kontrol S49 Mk3 keyboard controller and was trying to register it with Native Access. I got an error message of "An error occurred while processing your serial."
And yes, I double, triple, quadrupled checked (and more) that I was reading the serial number on the controller's display correctly (it matches the S/N printed on the bottom of the keyboard), and that my entries in Native Access were correct.
The error message seems a bit odd, in that it's not something like "Invalid serial number". Almost implies a database error on NI's side.