I'm taking this step by step.  So, first, I cleared out all EZ Skin3 installations.  Then I installed the Poser 11 version (good thing to point out that the Poser 11 and Poser 12 versions don't interchange).

Then I ran it from the scripts menu.  Well, I tried.  Same result.  So I tried to manually run the script from the Poser Files menu instead.  THAT worked.  At that point, I specified Custom Engines, imported legacy definitions and presets.  Shut everything down again.  Loaded P11 again, and tried to run EZ Skin from the Scripts Menu.  No go.

On a hunch, I ran P11 from the Start Menu instead of the Task Bar.  That was actually worse: it loudly declaimed "

Failed to load URL http://127.0.0.1:11530/nonVirtualAssets/ContentLibrary-1.9.11/release-1.9.11/ContentLibrary/ContentLibrary.html with error (-401)."

Summary:  I can only run EZSkin3 manually.  I am now going to install the P12 version and try the same experiments.