Oauth user-agent flow not working and gives error

I am getting a strange problem as i am trying to integrate oauth with my IAM(Keycloak) and testing a API-Connector plugin. After when user authenticate successfully and it try to redirect back to redirect URL, an alert message show on the screen:

I don’t understand this what actually it is. Currently my IAM is in dev environment and i am using selfsegined certificate. Is this because of selfsigned certificates OR something different?
Here is my IAM oauth settings and the API-Connector settings: