The error occurs if you focus in one of the date fields. Apparently, a datepicker is dynamically added at that point.
It looks like you're using some Wordpress plugin called Gravity Forms, which apparently uses either jQuery UI Datepicker, or some derivative thereof. Your page doesn't actually use/load jQuery UI. Somebody has taken the jQuery UI datepicker, and incorporated it into this WP plugin.
Who know what changes they might have made?
I can't easily see where they've reference the jQuery UI icons file. You need to find it and remove the protocol part of the URL, so that the protocol will match that of the HTML.
It is just a warning, in any case. The file loads anyway. (Unless some browser now is so strict that it does not permit the file to load, or users has set browser settings to not load insecure content.)
I suggest you check with the support forum for Gravity Forms.