I don’t know if my problem is the same, but the lightbox on a site I’m working on is also not working. I believe it was working before (this is one of those projects that gets passed around a lot), but is no longer working. I tried upgrading the plugin, and that also didn’t fix it.
I think I know what the problem is, but going through the plugin I was unable to get it change.
Then, looking at the lightbox setting, I saw that it was set to “thickbox”. After changing it to pretty much any of the other lightboxes, it started working for me.
The problem with thickbox is that it’s spitting out the class as “\”thickbox\”” which is causing the thickbox to fail loading. If you change the class to be “thickbox” in the inspector, it works properly. I’ll report this as a bug, but just figured I’d mention this. I don’t think OP was using thickbox as their lightbox choice.