That "bug" depends not on the Windows version but on the Mp3tag version. I think with newest version you mean 2.48. But there is a newer development build 2.48b:
Mp3tag Development Build Status
I was a bit quick to use the new scripting funtion $getenv() there, which gives you automatically the right program folder with $getenv(appdata)/Mp3tag/ (in fact, this new function brought me to the idea to make the web scripts out of export scripts).
When you have a older version, the scripting function is not recognized and you get a new folder as you describe.
I parse the HTML files with my script. API is used only be some of the search modes. Maybe I should switch to API completly, that would more stable. But I would have to set them up completly new. But I was already thinking about switching to API this time when discogs changed their script. I've had to change so much things with this freaky html code, maybe it would have been easier to switch to the clear structure of the API sites.