Hey there @loren.cahlander welcome to the community!
This typically happens when URL you’re redirecting to in your app is not listed as an Allowed Callback URL in the App Settings in your Auth0 Dashboard.
Are you able to confirm that the URL you are redirecting to post authentication is indeed added correctly? Aside from that, what happens if you define your redirect URL with a port and add that to your app settings as an allowed callback URL? Thinking http://localhost:3000/_codexch
Let us know!