The plugin requests the result URLs from WordPress core itself with the get_permalink(..) function. If the protocol is not correct, there is a good chance that either within the wp-config.php or within the database the site URL is not set correctly – it especially applies for cases where the site was transitioned from http to https. I recommend checking the site URLs in both wp-config.php and on the back-end as well to make sure it's indeed set correctly. The plugin does not affect these values, it serves the urls as returned from the WordPress core.
Alternatively this custom code can be used to make a correction if nothing helps at all:
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