Trying to give closure to this topic:
This is a very misleading situation where it seems Nifi is not running but it actually is. And you get ERR_CONNECTION_CLOSED or 'can't display page' etc. error when you try to hit the secure url.
Reason: Nifi is configured to authenticate client for connection and since there is not authentication provided the UI errors out.
Resolution: Import client certificate in your browser. This certificate needs to be trusted by the same authority as Nifi.
Once imported, close and re-open the browser (refreshing will not work). Given you have imported the certificates, you should be prompted to select a certificate to login.
This issue is not limited to IE. A similar issue with cached credentials/session may occur in Chrome. Restarting Chrome resolves the issue as well. To restart Chrome without losing current tabs, type chrome://restart in a new tab.