I have a Samsung Tab A 10.1 (T510) with the latest Android 10 and OneUI 2.0. I’ve been using Magic Dosbox on this tablet for quite a while now and never encountered this problem until about a week ago. The Android 10 update could be to be to blame, it was released a few weeks ago and I’m sure I never encountered this issue on Android 9. I’ve only come across this error by accident after hitting the home screen button by mistake.
If I move the files to internal storage I don’t have any issues so it has to be to do with the SD card. The other workaround is to enable SVN core and save the state when the error occurs, if I then reboot the game and load the state I have SD card access like normal. It seems purely to do with minimising DosBox while mid-game.
Let me install DosBox on my Android 10 Samsung A40 and see if it throws up the same problem!