Marketpress breaks the plugin editor?

I tried to go to the WP plugin editor on a site where Marketpress is installed, and the page came up blank. I deactivated all other plugins, and the page still came up blank. Then I deactivated Marketpress, and was finally able to get to the plugin editor. But then, when I chose Marketpress from the dropdown and clicked select, it took me to a blank page. (Blank except for the WP menus on the left side.)

Any idea why that's happening?