GSB hangs on launch after downloading challenge

I downloaded a challenge which described itself as having an unbalanced ship design that was very difficult. I assume this is a modded ship. However, GSB locked up after hitting the download button. I waited 10 min or so. used task manager to kill the process and restarted GSB, but now GSM hangs at the load screen with menu buttons, but I get no response. I have no way of identifying the modded ship. Waited 20 min or so before killing the process again, restarted, same results. Any ideas?

Delete (or backup) this:

MyDocuments\My Games\GratuitousSpaceBattles\web\challenges

and then try re-running the game.