Cropping Avatars lead to blank page

When I attempt to crop an avatar it simply leads to a blank page.

I get a 500 Internal Server Error from the request:
http://XXX.COM/wp-admin/options-general.php?page=blog-avatar&action=crop_process

The alternative upload also produces a blank page with no errors.

What PHP libraries are needed EXACTLY? I am running this on a very fresh server installation.

I installed PHP5-GD but that did not help. Any other libs that are required?

Thanks