Megamenu 1.4 causing site to crash

I have a Joomla 2.5 site installed at GoDaddy and just installed T3 v 1.4.

All works fine until I select the Megamenu tab in Templates to work on the MegaMenu. After that - the site crashes and posts a 404 error...

I'm working on the blank template....

Any idea what would be the issue?