URLs won't launch

I have a course where I created a custom resources slide with hyperlinks that launches from the top menu in a lightbox. I've done this before and it has always worked fine. In this course, however, all of the other triggers on the slide work except the Open file/URL with the link that launches the new browser window.

When I test the link from the trigger wizard it launches fine. When I preview it, the other triggers on the slide work but I do not get the popup indicating that links don't work in preview. And, when I publish the course the links don't launch. Yet, the same links launch from elsewhere in the course.

I'm at a dead end. Can anybody help?