FWIW, this is being emitted by the ReplayGain plugin in case that wasn’t apparent. (I don’t know why this error message is so uninformative; it should really have made that clearer.) So as a last resort, you can disable that plugin for now.
It is indeed mysterious that this would happen if you have python-gi and streamer-1.0 installed—I can’t think of anything else that’s missing. And, unfortunately, we don’t appear to log any details about what exactly went wrong when trying to load GStreamer (see the `_import_gst` method in replaygain.py).
I’ll look into adding more descriptive error messages/logging, but maybe others have more Ubuntu-specific insight?
Adrian