"Document has been changed by others" when saving a file

Upon re-starting computer, and re-opening several documents, when saving any restored file it now says, “Document has been changed by others.” This has never happened before. I am the only user of this computer. What does it mean? Does someone have access to my documents? Thanks.

Such a question is rooted in the filesystem and therefore needs the name of the OS. How did you restart the computer: cold start or warn start? I mean, did you really fully stop the OS or used some kind of “sleeping” (like what is the case with recent Windows versions to speed up restart)?

Please edit your question (don’t use an “answer” which is reserved for solution) to provide this additional information. Mention also LO version.

EDIT: Thanks, yes. It was a cold re-start. I shut it down completely. Awhile later, I started it again. There was no warn to start. Usually, however, I put it in hibernation (laptop). The OS is Win 10 Home 64-bit. The LO is 6.4.0.3 (x64).