Actually this was the exact error message of my 7.14.2 instance - "Oops! Error. Try again".
Problem solved, I was missing the native realm configuration in the Elasticsearch.yml.
Here is a link to the related documentation:
https://www.elastic.co/guide/en/elasticsearch/reference/current/native-realm.htmlhttps://www.elastic.co/guide/en/elasticsearch/reference/current/native-realm.html