its not strange that it works with the old version, that uses some default zbrush GoZ process and in the new version im bypassing that with that zscript to overcome some of the limitations of the default method.
The zbrush plugins work because you opening them usually from within zbrush, this script needs to be executed from the system since i run it from blender so your system needs to load it with zbrush.
That is the reason why it opens a text file because the default is set to notepad and not to zbrush.
you do not have that additional option in the open with context menu?
you can verify and change the default app also via the registry if you are comfortable doing that, you should find it in this location:
HKEY_CLASSES_ROOT\zbrush.script.1\shell\Open\command