Hi. Sorry for necroing the post but i have the same problem as OP, just started a new game in Ultra moon and get the following message when trying to open it in pkhex:
attempted to load an unsupported file type/size
File loaded:
H:\jksv\saves\pokemon_ultra_moon\UM\main
File size:
445440 bytes (0x6cc00)
and for you that are wondering, i'm not using an app for transferring, just plugging in the sd card in my pc.
Any help is appriciated.
main