The issue is likely that you have all the scopes in the developer portal UI checked off. You should only check the scopes that are absolutely required for the integration to work, and you should leave the 'Add action forms' scope unchecked since that feature has been sunset.
Additionally, you should make sure that you're only including scopes that you need and have access to in your authorization URL. I can see that the transactional-email scope is included in the example URL you listed; if your portal doesn't have access to the transactional email add on, you won't be able to authorize the integration.