I followed the instructions at this wiki entry, managed to build (pretty involved step, it was) and copied to target folder:

$ ll /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstspotify.so
-rwxr-xr-x 1 root root 92798088 Apr 13 15:40 /usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstspotify.so

Restarted Strawberry, logged into Spotify, added a song from the latter into the playlist in the former, still get this error:

Missing GStreamer plugin for text/html decoder (gstreamer|1.0|strawberry|text/html decoder|decoder-text/html)

What else do I need to do?