wp-admin - This feature is disabled

Hi,
Bit of an odd one...
Whenever I am on the home page of my website and click on 'Dashboard' it comes up with a white screen and an error message of 'This feature is disabled'.
However, at this point the url is http://www.rocketbuilder.io. If I add 'https://' in front of that url it loads the dashboard fine.
Odd one, I can work around it, but not ideal for a multisite I want to role out to the public soon.
Thanks.
Alex