I believe I follow what’s going on here, but please feel free to clarify if I’m off.
If you are having issues with your implementation of sign in with Twitter with your own Twitter app, then you should review our documentation on Callback URLs to ensure that you are whitelisting the proper callback URL in the apps dashboard and using the same URL in your request to oauth/request_token.
If you are having issues with someone else’s implementation of sign in with Twitter, please reach out to the developer who manages that app to have them make the adjustment that I outlined above.