Next.js provides a router which can be utilized via a component imported via next/link, a wrapper withRouter(Component), and now a hook useRouter().
When using any of these, it is expected they are only used for internal navigation, i.e. navigating between pages in the same Next.js application.
Either you passed a non-internal href to a next/link component or you called Router#push or Router#replace with one.
Invalid hrefs include external sites (https://google.com) and mailto: links. In the past, usage of these invalid hrefs could have gone unnoticed, but since they can cause unexpected behavior we now show a warning in development for them.
Look for any usage of next/link or next/router that is being passed a non-internal href and replace them with either an anchor tag (<a>) or window.location.href = YOUR_HREF.