devmode hangs on New Game

[smithfield]

I've made an addon which I've been using for over a year and I know its stable.
But I don't use the dev mode and I hadn't tried to upload it until today.

I followed the instructions on the wiki.
Entered devmode and clicked New Game.
Hangs.

I removed all of the addons from my addon folder and tried again.
Click New Game. Hangs.