The only thing I can suggest to you is just make the plugin ssl. There is no point in having both because on a non ssl site it would not cause mixed content issues. So just make it all ssl.
If anyone is still struggling with Mixed Content issues even after trying all the possible solutions provided on the Oxwall Forum, try this:
Why not just add this to your page settings "Custom Head Code" --> Go to Admin Dashboard --> Settings --> Page Settings --> Add this meta tag to your "Custom Head Code" section:
<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">
For a deeper understanding, please refer to the following: https://developers.google.com/...fixing-mixed-content