My other save files are from earlier versions, so I don't think (?) that is a fair comparison.

I started a new game earlier today, and had three successful autosaves on that one, so it's not necessarily an autosave problem, I think.
I've also checked the disc Factorio is installed on for errors ...

This happened twice this morning (first autosave of the day). After the first one, I quit to desktop, restarted Factorio and loaded an older save. As soon as it tried to auto-save again, the error re-occurred.

I'm not sure how to post this on the Factorio Forums as the game suggested (I don't have ...