Oh... This is a known issue — on newer Android versions, the file picker returns a special system path that our game can't read directly. We'll fix this in the next update.
Workaround for now: Instead of picking the file through the browser, manually copy your save.mss file to the game's internal folder:
/storage/emulated/0/Android/data/com.wildpear.magicacademy/files/
Then try loading from there. If you can't see the Android/data folder, use a file manager like X-plore or Total Commander — the default one often hides it.