This issue might occur if the client gets deleted in the API console. In order to resolve this issue, we would suggest you to navigate to Sign In Method under Authentication tab in Catalyst Console and then disable and enable the Sign In Using Zoho ...
If you are referring to the registration page(sign up page), you can design it as per your requirements. If you are referring to the login iFrame, as of now, we do not have the support for customising the same. We do have it in our pipeline.
You need to set the redirection URL in your application in your client-package.json file in order to redirect your application to that page after login. You can find the help documentation for the same here.
As of now, it is not possible to have a custom Login UI and still use the Catalyst Authentication feature. The ideal implementation of your custom logic can be determined based on your use case/business logic.