Plantuml diagram fails to display

[Ashley Kitson]

Just added plugin yesterday to our cloud instance. When page is displayed, the diagram isn't.  Console has a warning: 

Mixed Content: The page at 'https://zinc-digital.atlassian.net/' was loaded over HTTPS, but requested an insecure image 'http://zinc-digital.atlassian.net/login?application=confluence&dest-url=/wiki%2Fdownload%2Fattachments%2F77791247%2Fexampleuml.png'. This content should also be served over HTTPS.

and page just displays a broken image link. No other errors are being displayed

[David Benson _draw_io_]

Was exampleuml.png generated by the plugin?