Thanks for posting a link and turning on debugging. I tried to reproduce this issue on my test system and for me the thumbnail images were showing up correctly as HTTPS. So we'll need to do some digging to get at the cause.
We have the pro version and I checked the enable https option in the advanced tab. I also updated the tubepress.js file and the youtubeextension.php file to reference https://youtube.com instead of http://youtube.com.
Ticking that "Enable HTTPS" checkbox should be all you need to do; there shouldn't be any reason to edit any files manually (unless of course, there's a bug).
Let's try this. Please change the shortcode on your site from
[tubepress mode="playlist" playlistValue="5b4qDBro84rEx666zJNaOexxopgTUAMg" resultsPerPage="6"]
[tubepress mode="playlist" playlistValue="5b4qDBro84rEx666zJNaOexxopgTUAMg" resultsPerPage="6" https="true"]
You could brute force the solution by adding a line to tubepress-content/add-ons/youtube-black-bars-remover_4.0.0/classes/tubepress/bbr/impl/listeners/media/MqDefaultListener.php. Around line 94 change the code from:
But this of course is just a patch and isn't really getting to the root of the problem.
Let us know what you think! Thanks
Side note - keeping debugging enabled won't affect performance; it's just a relatively small privacy/security risk as you are revealing some internals of your server (filesystem paths and TubePress configuration). That's the reason we suggest leaving it disabled once everything is working properly.