Actually there are issues. 9.1D is ancient, obsolete and unsupported. Should you have the misfortune to run into a bug you will be out of luck. If you are running a business on 9.1D you are using unpatched software that is more than 10 years old. Just curious but how long has it been since your Windows OS was patched?
Major 9.1D issues that spring to mind include the infamous "workgroup performance gets significantly worse in the presence of multiple cores" bug, some fairly serious after imaging bugs, poor performance of 4GL string operations and some rather unhappy consequences to running with private buffers in use.
9.1E was released for a reason. So were the many service packs to both 9.1D and 9.1E. To say nothing of OE10.
Sure, 9.1D "works". So does Windows 95.