Insecure XMLHttpRequest endpoint: the content must be served over HTTPS

I’m trying to create a test on an Elementor page. This pop-up error appears when I click the Save button: “Error saving new test”

In the browser console, the log shows:
Mixed Content: The page at ‘https://www.mysite.com/wp-admin/post.php?post=56746&action=elementor’ was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint ‘https://www.mysite.com/wp-json/splitTestForElementor/v1/tests’. This request has been blocked; the content must be served over HTTPS.

In the Network tab, I see this information about the request:
Requested URL: https://www.mysite.com/wp-json/splitTestForElementor/v1/tests/

Response Header – Location: https://www.mysite.com/wp-json/splitTestForElementor/v1/tests

Any idea how to fix this error?