Please read my previous response, the problem is exactly what I expected. WordPress returns the incorrect ajax URL (with 'https'), because it thinks that the current site is using SSL. The plugin uses the WordPress code admin_url() function to parse it. Make sure to correct the site URL and that there is no rule forcing to redirect to the 'https' version in the .htaccess file.
As a bypass solution you can try this code:
This is however only a bypass, I would still recommend investigating why the site returns an incorrect ajax url.Best,
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