It sounds like you either have a corrupt slide, or some element of a slide in your project has become corrupted.
The easiest way I've found to locate the issue and resolve it is to open the project, and then create another project of the same dimensions. Once you have two projects showing in Captivate tabs, paste a few slides from the suspect one into the new one and try saving each time.
If the corruption stems from some object ID conflict (where two objects are fighting over the same ID) then pasting slides into a new project should get rid of that issue because Captivate renames all elements when copying and pasting. If the corruption is not elminated this way, then you will locate the issue as soon as you try to paste and save the corrupted slide/s into the new project.
Remember to always have the preference set to save project backups. Captivate 5x rarely corrupts, but it is still possible.
Thank you for responding. I was able to get around the error by copying slides into a new project. Hopefully I can figure out which is the corrupt slide.
One last question – I have verified in Preferences that Generate Project Backup is checked. If the program closes on me – where do I access that backed up file?
Your backup file is saved to the same location as the project file. But I believe that backups are only saved out when you close down the file. So if you end up having to use the backup, it's probably only going to get you up to where you were at the time you ended your previous working session.
There is another option for resurrecting projects that involves digging out the files in your cache: http://blogs.adobe.com/captivate/2010/09/recovering-the-project.html