Something was incorrectly saved. Can you describe as well as possible your procedure outside PrePoMax and the versions of PrePoMax you were using? Did you create everything in the same version (there is a bug that I fail to find).
To get the model working, you can try running the command history again. If you still have the .step file in the same folder, you can rerun all PrePoMax commands using: Edit → Regenerate History. If the .step file is in another folder, use Edit → Regenerate History Using Other Files and select the file.