SOLVED!
As I saw in another thread, someone who was having very similar issues had downgraded from 4.0 to 3.4 and it solved the problem.
I downgraded both Firmware and Engine program to 3.4 and tried the export again.
The SD card works perfectly now.
So for everyone trying to point fingers at SD cards, PC, OS, etc. No, it’s none of that. Denon broke something with their 4.0 update.
IF YOU’RE HAVING THIS ISSUE, DOWNGRADE FIRMWARE AND PC SOFTWARE TO 3.4. 4.0 IS BROKEN!