Keycloak with Azure AD | Invalid user crendentials

We are using Keycloak (hosted using Azure App Service) and Azure AD in our setup.

We have a App registration which has access rights to User and Directory.

We have configured the OIDC Identity provider by importing the OIDC configuration from the App registration. The client id and secret from the App registration have been applied in the OIDC configuration.

When we login using our custom login page which passes on the username and password to the Keycloak App service we see the following error message in the App Service log stream:
2021-01-08T12:43:29.139075603Z: [INFO] e[0me[33m12:43:29,138 WARN [] (default task-344) type=LOGIN_ERROR, realmId=TESTREALM, clientId=TESTCLIENT, userId=null, ipAddress=IP:PORT, error=invalid_user_credentials, auth_method=openid-connect, grant_type=password, client_auth_method=client-secret, username=TESTEMAIL, authSessionParentId=afc0de76-1754-4078-95e9-08f18c806b66, authSessionTabId=dtbYKBHr2WQ

Why are the user credentials invalid? Where can one look further?