This is usually not a fault of the save file, but of the . Resident Evil 4 is notoriously picky about the console region. If you are playing the NTSC-U (North American) version of the ISO but your BIOS is PAL (European), or vice versa, the game may refuse to save or load.

Internal Storage → Android → data → xyz.aethersx2.android → files → memcards

If you are looking to save your progress, transfer data between devices, or fix a corrupted save, this deep write-up covers everything you need to know about managing save data on AetherSX2.