Big Red X in Story View

We had a couple of instances when this occurred and came up with the following workaround:

1. Close Storyline as soon as the problem occurs.

2. Re-open Storyline and open and  go through the slides one by one, clicking on each slide until the big red cross appears, and note the slide number.

3. Close Storyline again and reopen the project - remain in story view, do not open any individual slides.

4. Delete the corrupt slide from inside story view.

5. Close and re open the project, you should now be able to insert a new slide in place of the deleted one and your project will be free of corruption and the dreaded red cross!

I'm not sure if all of the above steps are necessary, but this has worked for us a couple of times now, hope this helps someone else.