Hello, @trainr. Thanks for reaching out and welcome to our community.
Usually, the error page explains what is the problem. Sometimes is as clear as the following one, sometimes the error is a little bit more hidden.
On the example above, you can see that the page is stating that
http://localhost:3000/callback is not in the list of allowed callback URLs. In that case, I would have to evaluate what I have done wrong (perhaps I forgot to add this URL to the Auth0 Application page, or perhaps I mistakenly configured the Auth0 library in my app).
If the page is not that clear, it will probably add some info on the Technical Details section on the bottom.
As you can see on the page above, after clicking on the See details for this error link (now called Hide …), I will get some info about the error.
Please, can you check your error page and see if you find valuable info there? If not, let me know and I will help you investigate.