Any chance your save game folder somehow got set to "read only." I'm pretty sure this was something that Windows 10 was doing randomly to a lot of people at one point during the upgrade process. Haven't heard about it much recently, but if you hadn't been in there recently, perhaps it is still a remnant of that upgrade problem from a while back.
You'd have to navigate to that folder via windows file explorer and check it out that way. No way to check for sure in-game.
This post is all applicable only if you did indeed upgrade to Win10 of course!