I'm not too experienced with the Android version of PPSSPP, but I know that since 4.4 (which the Z2 should come with) Android handles SD cards and their contents differently, as well as there being some potentially relevant access modes that might need to be switched to/from.
Have you tried copying the files to the internal storage, instead? That might be worth a shot, if you can get into there and have enough space.
PS. The path in that error of yours is probably supposed to be: disc0:/PSP_GAME/SYSDIR/BOOT.BIN
Have you tried copying the files to the internal storage, instead? That might be worth a shot, if you can get into there and have enough space.
PS. The path in that error of yours is probably supposed to be: disc0:/PSP_GAME/SYSDIR/BOOT.BIN