SSH tunnel for VNC suddenly stopped working, error "channel 3: open failed: connect failed: Connection refused"

I set up VNC on my Droplet using the instructions here. I then created an SSH tunnel from my Mac using this Terminal command:

ssh -L 5901:localhost:5901 vnc@IP

Initially this worked well, after entering my password a connection was created and I was then able to use Screen Sharing to connect to the machine using localhost:5901.

All of a sudden though, the connection dropped and the Terminal started outputting:

channel 3: open failed: connect failed: Connection refused

every couple of seconds. Disconnecting and reconnecting, after asking my password, resumes this same behavior.

How can I fix it? It seems very odd that it started happening with no action on my part.