Getting “mixed content” issues in the dashboard

I am trying out this plugin for the first time. On the “CSS & JS Manager” tab in the WordPress Dashboard step 1 (fetch assets from the homepage) is not completing.

When I go to the Javascript Console I see the following errors

Mixed Content: The page at 'https://xxx.yyyyy.com/blog/wp-admin/admin.php?page=wpassetcleanup_assets_manager' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'https://xxx.yyyyyy.com/blog/?wpassetcleanup_load=1&wpassetcleanup_time_r=1645003186371&_=1645003186252'. This request has been blocked; the content must be served over HTTPS.
script.min.js?wpacuversion=1645000482:2 Uncaught TypeError: Cannot read properties of undefined (reading 'replace')
    at Object.complete (script.min.js?wpacuversion=1645000482:2:5874)
    at c (jquery.min.js?ver=3.6.0:2:28327)
    at Object.fireWith (jquery.min.js?ver=3.6.0:2:29072)
    at l (jquery.min.js?ver=3.6.0:2:80029)
    at XMLHttpRequest.<anonymous> (jquery.min.js?ver=3.6.0:2:82330)
complete @ script.min.js?wpacuversion=1645000482:2
c @ jquery.min.js?ver=3.6.0:2
fireWith @ jquery.min.js?ver=3.6.0:2
l @ jquery.min.js?ver=3.6.0:2
(anonymous) @ jquery.min.js?ver=3.6.0:2

I had a brief look at the plugin’s code and I can’t see why this is happening.

I have confirmed the following:

• get_admin_url returns the https version of my website
• $_SERVER['HTTPS'] === 'on'

Any other suggestions?

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