When you run a dynamic cloth simulation, it will leave a file called *.DYN (filename matches the name of your scene file).  This DYN file contains the morph data for your cloth simulation.  I've never saved any PZ3 files in my Runtime directory, so I doubt that's it.  If you rename the scene file outside of Poser, it may show the kind of thing you're seeing; from experience, if you re-save the file within Poser 7 after you have run a dynamic cloth sim, it won't error but you do need to re-run the simulation - Poser 6 didn't make you do this as I remember, I think it automatically copied the DYN files as you saved the scene file.

edit: small correction, filename for .DYN files is the name of your scene + the name of the cloth simulation, for as many cloth simulations are set up in your scene.

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