New Auth - Blank Screen when authorizing

Very much a newbie, I'm trying to create a custom connector for Power Automate, managed to get this working using the older method (although it wouldn't refresh the token) but have hit a roadblock with the new method. 

I have input as before and when attempting to authenticate and grant permissions the login box shows up completely blank, literally nothing loads. 

The URL used to request is below:

https://identity.constantcontact.com/oauth2/REF/v1/authorize?client_id=API_KEY&response_type=code&redirect_uri=https%3a%2f%2fURL%2fredirect&scope=contact_data&state=STATE_uk-001_azure-apim.net

Capital and bold indicates replacement of original value.

I have tried replacing the full stops in the redirect with html but still nothing. 

Any clues would be really helpful