I was doing a restore, and had to abort it. This resulted in a LXC container that was locked, with no obvious option to unlock it.

Toop me a minute to find the fix:

1) In the web interface, go to node (first item under the "datacenter")
2) Open the console there
3) in the console unlock the LXC like so (where "300" is the container ID number):

pct unlock 300