I have given up on Ajenti and I am now running ISPConfig version 3. I can highly recommend it.

Whatever I do, the server will not respond to port 8000 with https.

I have done all of the above suggestions, even tried changing the port to 9000, but I can not access ajenti on any port with https, nor with http.
I am running a fresh install of Ubuntu 14.10 with LAMP and OpenSSHserver; for the rest it is all standard.
During installation there were no errors.