Hi Stuart,
I also answered your other post here, and I wanted to share the info for Chris as well.
Have the learners tried to refresh the browser page? It sounds like it may be stuck on the relaunch as it's a new version of the course.
When you update (overwrite) existing course files in your LMS or web server, learners who previously started the course won't be able to resume where they left off. They should start over at the first slide in the updated course. You'll want to keep this in mind when updating courses that have already been delivered to learners.