Preview not working

Hello, hoping you can assist.

We have a module which has just been reviewed (again), it was originally written in 2018.

Other modules are fine, but this one refuses to preview. Well the preview goes up but when you hit 'start' button it goes out of preview mode and back to the standard view when you are editing.

I have tried importing it into a new project, but it does the same thing. The first slide will preview singularly, but the rest of them appear to be on strike!

Before I try something very long winded, could someone please have a look to see what might be the issue? I don't wish to upload here publicly though.

Appreciate any help you can offer, thank you.