I made some code changes to prevent data loss, but I could not reproduce the problem, so I am not sure it is a 100% solution. I think the problem occurred because a save attempt failed.
To add an additional safeguard, the .pmx files from an older PrePoMax cannot be overwritten. If the user saves the model to a file that already exists and the file is an older .pmx file, the older file is backed up with a version extension.
So the backup happend when the file is saved and not when the file is opened.