When closing lightbox, blank slide shows.

I can't reproduce the issue of landing on a blank slide since I'm unsure how you're returning. Personally I found the course design confusing. The lightboxes in particular seem to be creating a number of wrenches. It took a few times restarting the course to realize the Continue button on 6.5 wasn't closing the lightbox you'd created but rather navigating to 6.1 inside of the lightbox. So 6.1 was showing itself in a lightbox. That may or may not be related to what you're encountering. I was able to break the loop I was stuck in by changing 6.5's Continue button to only fire a Close lightbox trigger, and then change the trigger of 6.1 that shows the Next button:

The Next button was showing in your original version because its trigger fires when 6.1 starts, and since 6.5's Continue button was launching a second, new instance of 6.1, the trigger would (re)fire, but then the user would be trapped in an endless loop of lightboxes I think. Personally I find putting things on layers easier to compartmentalize than lightboxes, so maybe those will help.

If adding the Close lightbox trigger to 6.5 and updating 6.1's Next button trigger don't resolve your issue, be sure to reply back and maybe we can better dig into what you're specifically clicking and where and when.

Additionally, can I recommend when asking for help with a course that answer keys be provided for any required quizzes? I did find both of them to be unexpected points of friction in my investigation. I'm also curious at a course that tells learners they're wrong but doesn't provide targeted feedback as to how. Community best practices of learner experience design show that leaving out that feedback can significantly increase learner frustration, turning your well-intentioned quizzes into more of a guessing game.