Ran into this issue today on a couple of my machines. It started with my laptop losing its Tomboy ssh sync configuration. I went into Tomboy’s preferences and tried setting sync up again. Whenever I would click save I would get a generic message that contained “fuse: mountpoint is not empty”. I then attempted to sync using one of my desktop systems and I got an error indicating that the service is unreachable. After digging around in the logs and online, I discovered that running the following command from your home directory will resolve the issue.
rm -f ~/.tomboy/sync-sshfs/lock
It appears as though one of the recent syncs failed and orphaned the lock file on the clients. This has happened to me on the server as well and I have resolved it by running rm -rf lock on the servers lock folder/file. As always, YMMV but these steps resolved my problems.