I am using this sample app https://github.com/pozil/salesforce-react-integration to create a React app connected to Salesforce.
I have created a Connected app inside my Salesforce, enabled OAuth and added a callback Uri that is identical to the one in the sample app. I have also replaced the consumer key and secret in the sample app for the ones in my SF Connected app, yet after entering and submitting my Salesforce login details, net::ERR_CONNECTION_REFUSED is being thrown on the http://localhost:3000/auth/callback.
Any solution would be appreciated.
Connection refused could be a few things. Have you considered Login IP ranges etc?
I would recommend reviewing all login and security configurations.