[date] [hostname] kernel: sonewconn: pcb 0x[hex]: Listen queue overflow: [#] already in queue awaiting acceptance ([#] occurrences)
Seen some people talk about the web shell being a bit finicky. I had opened it up many times since it rebooted last, and had the console messages in the footer enabled (System > Advanced > Show console messages in the footer). After some time, attempting to start the shell would show the "Loading" message for a few seconds before the window disappeared and the error message appeared in the footer.
Couple people said to restart FreeNAS. That wasn't an option for me.