It took me a second to understand the issue so I will take the time to explain it for those who went through the thread and don't see a solution.
Correct me if I'm wrong but I think the problem started when Webflow removed js that wasn't being used in a project. So if you never used an actual lightbox in the project then the java script for the lightbox wouln't load. The work around as Koen said is to add a lighbox into your project page but set the height to 0, I had to set height on the image to 0 as well. That way it doesn't physically appear on the site but the java script loads.