This is Peter Newman from Uformia. We ran into this issue with our plugin on one persons machine. They are running Windows 8 (not 8,1) but appeared to have the same issue.
We identified the issues as caused by an entry in the Windows registry:
In the registry folder:
There is an entry:
“C:\Program Files\Rhinoceros 5.0 (64-bit)\System\Rhino.exe”="$ IgnoreFreeLibrary<symvol.rhp>"
(Where symvol.rhp is our plugin.)
Deleting this entry allowed the plugin to install/load correctly. It appears our plugin triggered some Windows compatibility behaviour somehow (maybe a crash during loading or unloading or something), which then got in the way of the process Rhino uses to load plugins.
Hopefully this helps.