jayseventwo on "[Plugin: WP-Invoice - Web Invoice and Billing] 404 error on First-Use Setup"

Hi there, this looks like just the plugin i need, but when i try to save all settings on initial setup (paypal details etc) i get a 404 error.

I have tried turning off all my plugins but it still does not work.

The page with the 404 error is:

http://www.myurl.com/wp-admin/admin.php?page=wpi_page_manage_invoice

Any help would be greatly appreciated.

http://wordpress.org/extend/plugins/wp-invoice/