Usually, the issue is not actually with the ...\webagent\win64\bin\HttpPlugin.dll library, but rather it is caused by dependencies of HttpPlugin.dll that are not loaded properly.
Using Procmon, we are able to see what libraries are being loaded when the Web Agent tries to initialize. Below are some examples.
We can see that HTTPPlugin.dll is properly loaded from the ...webagent\win64\bin directory.
Now we see the agent trying to load smvariable.dll. The proper load should come from the webagent\win64\bin folder as we are using the 64bit web agent. From the image, you can see that the w3wp process is actually loading smvariable.dll from the Policy Server bin directory. This is not correct.
Here is another load that is found and loaded in the Policy server bin directory. This is also not correct as it should be loaded from the webagent\win64\bin directory.
If we check our PATH environment variable, we can see that webagent\win64\bin is listed AFTER the policy server paths. This is causing our problems.
Path=D:\Program Files\CA\webagent\win32\bin;D:\Program Files\CA\webagent\win64\install_config_info\lib;D:\Program Files\CA\siteminder\bin;D:\Program Files\CA\siteminder\bin\thirdparty;D:\Program Files\CA\siteminder\lib;D:\CA_JRE\bin;D:\Program Files\CA\webagent\win64\bin;C:\Windows\system32
This KB only refers to one of the many possible reasons for seeing the "ADMIN: SmDSOLoadLibrary failed to load plugin 'D:\Program Files\CA\webagent\win64\bin\HttpPlugin.dll'." error message.
For other use cases, please check out the following KBs:
52514 - Rouge DLL files in windows system folders will cause the webagent to fail.
53697 - Unable to load HTTPPLUGIN.DLL due to incompatible DLL files.