I'm getting a strange error on my live WordPress website. On the admin homepage, I'm getting a mixed content warning that points to the doctype. If I copy this entire website and database to my local machine, I'm unable to produce the error. It only occurs on the home page of wp-admin, and prevents other things from working - for example, when hovering over Posts or other links on the left-nav, I don't get the sub-navigation on hover - but if I right click, they appear.
The warning seems to point to itself:
Mixed Content: The page at 'https://domain.co.uk/wp-admin/index.php' was loaded over HTTPS, but requested an insecure script 'http://domain.co.uk/'. This request has been blocked; the content must be served over HTTPS.
This error started to appear after upgrading to WordPress 4.9.