Are you using a Static Page for the Front Page (
WP Admin -> Settings -> Reading -> Front page displays)? If so, is that set to the same page as your Membership Options Page? (It probably should be... I assume the front page isn't
supposed to be displaying a 404 page, right?)
If you're not using a Static Page, or if it's already set to the same page as your Membership Options page, then that tells me you have a problem with the PHP code in your theme's
index.php file which is then generating a call to the 404 page.
The problem with the 404 page displaying the sidebar can be resolved by
Creating an Error 404 Page and making sure that it doesn't include a call to the sidebar.