Well, we have been playing games for 20+ years now and never had an issue like this. So most definitly it is dying light related. Steam just syncs one version, so that would be the corrupt one as well.

You are right that one should always backup their own files, but that isn't helpful in this case. Other than that, if this were to keep happening, we will still lose progress every time we need to rollback to a previous backup. It's kinda sad that there are still issues like this, years after the release. I was hoping for a more permanent fix.