Fixer theme - 500 internal server error on activation


I'm new to WPMU DEV.

Have installed the Wordpress plugin, and am easily able to activate both the Scribe & Spirit themes, but EVERY time I activate the Fixer theme I get a 500 Internal Server Error.

Note that I have tried installing from both the dashboard and from uploading the zip file, and I've even deleted and completely re-installed Wordpress in a different directory and I still get exactly the same result (500 server error).

This is on a completely fresh install with no customisation or existing data to conflict with.

What is going wrong?