Something changed when you saved it last... change is inevitable, except from a vending machine. I also recommend going back and starting to work on the previously saved version rather than spend time trying to diagnose the corrupt file. Use the Save feature more often, yes, you will have more pz3 files crawling around but once you finish your project you can get rid of them.

Restart your computer, don't run anything but Poser, focus on your animation project.

PS: Okay, you could mess with the compatibility settings and set them as seen below, but like I said your wasting precious time you could've used to progress.