Blocked-Mixed-Content when trying to change forum subscription

Ember tries to make a request to an XHR endpoint of the old forum that is not served over HTTPS.
Console entry:

Mixed Content: The page at '' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint ''. This request has been blocked; the content must be served over HTTPS.

This makes it currently impossible to change the subscription status. Additionally the current status for all forums is watching.

This issue persists across forums.
Seen in Firefox and Chrome, macOS

Edit: Changing the status in the general site setting works fine.