Is this in every game? Does it happen if you create a new (empty) game? I know there was an issue with Tags on characters making insecure connections, but I think that has been fixed now. Did you get any images in your game from a search within Roll20 that you dragged to the VTT? When you do that, your character, handout, or map is just a pointer to the original image on a website somewhere, and that might be a non-SSL connection.