I’m really happy the legacy mode worked for you. Thanks for the info.
I can’t speak to the specific combination of Android version/HTC phone you’re using, but the behavior of the bluetooth libraries definitely varies between manufacturers. It makes it quite difficult to support all these Android devices… This is why I made the “legacy mode” a check box the user could configure instead of just running a check in the background and forcing legacy mode on Android versions <5. On some 4.4 devices the accelerated upload works fine, on some 5.0 devices it fails. Honestly BLE on Android is a bit of a mess from a developer's perspective.
Sorry I can't give a more clear answer, I'm happy the legacy mode worked for you. Please let me know other problems as they arise.