It works fine with the NPClient64.dll which officially comes with opentrack (
https://github.com/opentrack/opentrack/releases).
One of the problems is that some of the instructions (found in the net) on how to use opentrack's NPClient64.dll are plain wrong, confusing, or both. It is as simple as it could be:
All you should need to do is extract NPClient64.dll to the same folder where NPClient.dll is