Hey! I know this is a bit of a late reply here, but for future googlers and people like myself that are looking for a solution, this issue is caused by two things:
1) The plugin doesn’t detect the http/s scheme of the site before doing the redirect, and
2) because of the form using ajax submission.
To fix this, you have a few options. You can disable ajax submissions by adding the following to your functions.php file:
define( 'WPCF7_LOAD_JS', false );
or installing this plugin to disable it with a checkbox:
https://www.ads-software.com/plugins/contact-form-7-extras/
or nicely and politely bug the author to check it out ??
