Run Modal fails – tries to load over http and is blocked

I’m not sure why when I choose to run in Modal mode it never loads and just shows a white modal area. Inspecting the browser console I see an Mixed Content error:

Mixed Content: The page at 'https://mydomain.com/wp-admin/admin.php?page=ari-adminer-run-adminer' was loaded over HTTPS, but requested an insecure resource 'https://mydomain.com/wp-content/plugins/ari-adminer/adminer/wrapper_adminer.php?username=secret_hash&db=mydatabase'. This request has been blocked; the content must be served over HTTPS.

Additionally, looking at the code in the inspector I can see the iframe has a relative url which is also using the https protocol as the page the iframe resides within is loaded over https and any relative urls defined will use the base protocol and domain of the containing page.

src="/wp-admin/admin.php?page=ari-adminer-run-adminer&action=run&noheader=1&id=0"

I can also see that all of my scripts and styles are loaded with https urls.

Both my WordPress Address (URL) and Site Address (URL) are set to https as well. My whole site is https, front end, back end. I’m not sure why this plugin refuses to follow that protocol.

Opening in a new window uses the https url and works however.