I almost posted that I couldn't see the Subsonic drive, before I realized that I am using filtered folders and the drive was just hidden. Anyone else using filtered folders, make sure you add the Subsonic drive to your list.
The library has been loading for the past 40 minutes or so and still nothing is displayed in the folders. Just thinking out loud here, but I have many GBs of videos in my SS library alongside the music (SS supports streaming video playback too) - could that be an issue?
EDIT: Added below info
I have my SS library split into 3 folders: Music, Videos, and Downloads. I was hoping to troubleshoot my above "theory" so I tried to change the plugin settings so the library was looking just at /Music (also tried Music, /Music/, and Music/) but I keep getting unknown error popup.