What could be a reason of failure with loading library?
Other libraries is possible to load by direct command, but CnoidPythonPlugin.dll still is failed.
One possibility is that Python libraries you installed is 32-bit version.
Choreonoid only supports 64-bit Python on Windows, and similar problem was reported due to that.
Moreover, I would like to ask you which version of Choreonoid you try to use (e.g., 1.7 or latest on GitHub).
It should be noted that the developer team strongly recommends to use Python 3 with the latest Choreonoid.
(We have verified that Python 3.9.4 (64-bit) works well.)