500 error & "Unable to issue redirect for OAuth 2.0 transaction" - using "@auth0/auth0-react" on localhost

This suggests to me that there is a misconfiguration in your application’s settings in Auth0. Unauthorized can mean that the client id is incorrect, the applicaiton is using a grant it is not registered for, or something similar. Can you DM me your tenant name and client ID so I can take a look?