Because this has happened more than once and I keep wasting time figuring it out, I will post it here.
If I get a 503 error from subsonic or a message from rutorrent that it’s not connecting to rtorrent, one of the possible problems (check to make sure services are running first, of course; then check that ZFS filesystem is up) is that a bad shutdown has fouled them up.
Shut down subsonic service
Go to /var/subsonic and rename the db folder
Start subsonic and let it recreate the db folder
Copy an older version of subsonic.script into the new db folder (this has all the user settings, etc.)
Note: you may have to hunt around. The most recent subsonic.script file could be damaged.
Stop rtorrent service
Go to RtorrentSession folder and delete the rtorrent.lock file