I've just recently updated from 6.8.22 to 7.14.2.
I configured the username and password of Kibana in the kibana keystore.
My Elasticsearch cluster is up and running I can send requests via curl.
I try to authenticate with the built-in user elastic.
I suspect the login screen is being served from a Kibana 6.8 instance. The "Oops" message appears in the 6.8 branch but not the 7.14 branch: https://github.com/elastic/kibana/blob/6.8/x-pack/plugins/security/public/views/login/components/basic_login_form/basic_login_form.tsx#L207
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: