Activating Plugin leads to 404 Error

I went to install and activate Contact Form 7 (v 1.7.7.1). I clicked activate from the Plugin Management page and it went to a 404 Error page:
http://vuhoops.com/wp-admin/plugins.php?activate=true

Now when I’m in the dashboard and click on the Plugins tab, it goes to:
http://vuhoops.com/wp-admin/plugins.php
… a 404 Error page.

If I delete the plugin folder I can access the Plugin Management tab (and everything is fine and dandy). It is also happening with the Gig Calendar plugin.

Any idea on this one?