There are a bunch of situations that could cause this to happen.
If this was a published project I would say that you may not have the JSON mime type turned on in your web server or LMS, however, you did say preview.
Sometimes clearing your preferences may help. Navigate to the following folder...
Windows C:\Program Files\Adobe\Adobe Captivate 2017 x64\utils, or
Mac OS /Applications/Adobe Captivate 9/utils/
Run the file called CleanPreferencesWin / CleanPreferencesMac
This will reset your preferences back to default and may correct any issues.
Another possibility is that perhaps you have used a reserved or incompatible name for one of your objects, variables, quiz pools, or advanced actions. One way you could troubleshoot this is to copy all your slides into a new project and see if it then runs. If so, it could be a naming issue.
Another possibility is a corrupt question slide. You could try a "save-as" where you delete the first half of slides first and see if that works, then the second half and so on until you have isolated the corrupt slide.