Windows version 5.1.7-1.
I was given a small project to rescue, where the schematic was rejected as not a schematic.
I had a look with Notepad++ and the .sch-bak file was fine, but the .sch file was the correct size, but all “Null”, which is 0x00, so the file had been binary zeroed
Unrepeatable, of course, and I have not seen any reports of anything like this
OK, just a wild guess because nobody else has answered yet: In my experience, while it could be in an app (perhaps some kind of race or locking/access condition with another app), this more likely points to disk or hardware failure (or disk full?), something is wrong with the OS, or indirect user error.