To avoid any interference with the ogg vorbis files, my plugin only considers files with the extensions .opus or .opu. Try renaming the file and let me know; the plugin will in no cases be loaded if the file extension is .ogg.
Winamp 5 only knows ogg vorbis files and thus anything with the extension .ogg is assumed to contain a vorbis stream, in your case it contains an opus stream, the "proper" way to name those files is actually .opus, same container but different stream.
When I made experiments with various files it appeared that some opus streams would not be decoded by libopusfile, when they would work just fine with other decoders such as the one within Mplayer.
As I did not want to take time to try other decoders I just use ffmpeg to fix the file with the command:
ffmpeg -i broken.opus -acodec copy unbroken.opus
In case renaming the file does not work, try this...
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