Exporting Captions to Microsoft Word 365 fails

The first thing I would do is to test it out with a simple and relatively small project (just create a new project with the same theme but make it just a few slides long).  If it works there is a good chance that it could the size or something  else about the offending project itself.

I ran into a similar circumstance as yours during my time using Captivate 9, or more specifically on a responsive project that used five breakpoints. Each text object is duplicated for each breakpoint so the resulting export was five times larger than a non responsive project would be. I used a process of elimination to find out where the true problem was with my project.

To help narrow this down you might try saving copies of your project with the first or second half of the slides removed then attempting to export with the smaller halves. For arguments sake, let's say the second half of your slides exports fine but the first half has the same problem as the full project, you've narrowed down the offending slide or text object within to a smaller selection of your project. Repeat this process until you can know which slide or maybe even which text object is causing the hang up.

In my case I had one question slide that was corrupt. When I recreated that slide from scratch and delete the original it expired just fine.