Supporter Plugin + Premium Themes = Interternal Server Error

Hey guys,
I have been setting up all of the options for the supporter features using the supporter plugin.

Everything was going fine, until I logged in as a test user and found that if I clicked either of the "premium Themes" links I get an Internal Server Error.

I checked the server error log and the most recent one is this:

[Wed Dec 30 11:48:48 2009] [error] [client] Premature end of script headers: themes.php, referer: http://URL/wp-admin/plugins.php

Now even if I am logged in with the admin account I get the error.

Any thoughts?