If you've received this message like I did, you might not know where to start, or what to look for. I've captured a few of the ones I've found, to save you a bit of the same potential headache. 

If you recall adding a YouTube video, or <div> code to any of your lectures, consider checking those course(s) or lesson(s) first to see if you notice any issues. If you do, make note of them so you can make changes later. 

Most of my lessons include content I've uploaded to teachable, so they won't be a problem. But some have YouTube videos, and were added more than 2 years ago. Those are the ones I'm checking first. 

Here's the steps I'm taking to double check things & some examples of what needs to be fixed.

First, go to Admin > Settings > SSL and click on the "preview with SSL" button so you can view what your site will look like in SSL mode.

Your site will now display with https at the beginning like this:

Go to one of the courses you might suspect would have problems. Click on any of the lessons to see if it previews the way it usually does. It might, but it might also look like some of the pictures below.

Here are some examples of the problems on my site. 

Blank content

This one is just blank - there should be a video here but I see nothing instead

Naked Link Displaying

This youtube video link is displaying, instead of the actual video

Code Displaying

This one has some <div> code displaying where there should be a video 

To fix the lessons with code problems

Blank Content

When I go to edit the firsts lesson I can see it was an iframe YouTube video. (looks like this)

Add an "S" to the "http://" string (assuming those sites are running in https).

 Click Save and preview to see if the content displays as it should. It is in my case, but if I hadn't converted my blog to https already, I would have left the blog links as they were.

If you're working on several problematic lessons within the same course, use the right /left arrows while in admin mode. It will make it faster to navigate between lessons. 

Naked Link & Code Displaying

I think these may have been a problem before, and I'm just noticing today. They were both real coding issues The first one just had the youtube link - which works well in WordPress but not on Teachable. The second one had the code pasted into the rich text editor, not the code block (both are fixed now)

This is what the text block looks like in this lesson

Here's what it needs to look like 

The biggest thing (for me, anyway) in each lesson is to add the S where it was previously http://

If you have any other scenarios, let me know and I'm happy to help dig in and see if we can find a quick fix.