When you see a 404 page as your new tab page, the browser extension you have installed is configured to load a page that doesn't exist anymore. This can happen after you have a deleted a page on start.me.
How to fix this:
1. Go to the extension options page (in Chrome: right-mouse click on the icon in the top)
2. Select a different page in the dropdown "Which page do you want to use as your homepage?"