And a follow-up: I copied the iso to my internal sd, and setup the dos game directory on the internal sd, and it worked. I also renamed the iso to nba95.iso (no idea if that does anything, but the file name was really long).
So there’s something weird going on with external sd card support in Kitkat. Btw, I also tested with AnDosbox, and got the same bug. So it’s not Magic Dosbox specific, but rather some kind of general OS vs Dosbox conflict.