I must have mistaken the name for what is in the plugin, sorry.
But still, this problem occurs when trying to access playlists on the Subsonic server (not locally, those work).
It works fine in 3.3 releases (tried with the latest one I could find, Update 6), but not in 3.4 releases of MusicBee.
Something must have changed in how it handles this case, but I don't know what.
In 3.3 Update 6 (works as expected):
In 3.4 (no content shown in the playlist):
In both cases, the plugin and back-end subsonic server (with the playlist) are the same.