If any of the Portfolio page links result in a “Page Not Found” error, this can be fixed by disabling and re-enabling the Portfolio content type. Go to Settings → Writing → Custom Content Types, uncheck the Portfolio Projects option, save your changes, and then re-check the Portfolio Projects option and save your changes again.
Note: This setting is in the WP Admin dashboard, which you can reach by adding /wp-admin to the end of your site address (e.g.: yourgroovysite.wordpress.com/wp-admin)