I just find out that the problem is not directly related to the saving/loading operations.
It has to do with the fact that I was trying to save a file in the _exit_tree() function. It works on Mac and Windows, but not on Android.
func _exit_tree():
saveGame()
Here is my solution :
func _notification(what):
if (what == MainLoop.NOTIFICATION_WM_QUIT_REQUEST
or what == MainLoop.NOTIFICATION_WM_GO_BACK_REQUEST
or what == MainLoop.NOTIFICATION_APP_PAUSED
or what == MainLoop.NOTIFICATION_WM_UNFOCUS_REQUEST
or what == MainLoop.NOTIFICATION_WM_FOCUS_OUT):
saveGame()
get_tree().quit()
func _exit_tree():
get_tree().notification(MainLoop.NOTIFICATION_WM_QUIT_REQUEST)