The Windows Font Folder at C:\Windows\Fonts does not seem to allow the Opus filter bar to work. How come?
It works ok for me. Just the Fonts folder? Have you got real time filtering turned off?
Settings / Preferences / File Displays / Filter Bar / Real-Time Filtering
You're viewing Fonts as the virtual folder, handled by Explorer on Opus's behalf, so Opus-specific things like the quick filter will not work.
You can set Opus to show the real underlying directory instead of the fonts virtual folder, but you probably wouldn't want to as the font names would be replaced by often cryptic filenames that exist behind the scenes.
Using the search field may work (I'm not 100% sure off the top of my head).
I understand what you mean by the cryptic font names. I would like to know how to get Opus to display the real underlying folder.
The bottom half of Preferences / Folders / Virtual Folders is what you want.