Self-hosted JIRA server does not support authentication with API tokens. Case's JIRA connector uses basic authentication as an authentication mechanism. So, if you put your username in the email field and your password in the API token field the connector can push incidents to a self-hosted JIRA server.

The labeling of the two fields should be improved in a way to make it clear to the user that she can put the username and the password instead of the email and the API token.